Protein Metabolism

Dr. Arvind Kumar

Sources of Amino Acids

  1. Dietary proteins – obtained from food and digested into amino acids.

  2. Degradation of body (tissue) proteins – due to normal protein turnover.

  3. De novo synthesis – synthesis of non-essential amino acids from metabolic intermediates of glycolysis, TCA cycle, or pentose phosphate pathway.

  4. Transamination reactions – interconversion among different amino acids.

  5. Ammonia assimilation – formation of amino acids from ammonia and α-keto acids (e.g., glutamate synthesis).

 

Catabolism of Amino Acids

  1. Transamination – Transfer of amino group to α-ketoglutarate forming glutamate.

  2. Oxidative Deamination – Removal of amino group from glutamate to release ammonia (NH₃).

  3. Ammonia Disposal – NH₃ converted to urea in liver (urea cycle).

  4. Carbon Skeleton Utilization – Converted to TCA cycle intermediates for energy, glucose, or ketone body formation.

  5. Classification:

    • Glucogenic: Yield glucose (e.g., alanine).

    • Ketogenic: Yield ketone bodies (e.g., leucine).

    • Both: e.g., isoleucine, phenylalanine, tryptophan.

 

Transamination:

Definition:
Transamination is the transfer of an amino group from an amino acid to an α-keto acid, forming a new amino acid and a new keto acid.

Enzyme:
Aminotransferase (Transaminase)

Cofactor:
Pyridoxal phosphate (PLP) – derived from Vitamin B₆

Mechanism of Transamination:

  1. Step 1 – Formation of Schiff Base (Aldimine):
    The amino group of the amino acid reacts with the aldehyde group of pyridoxal phosphate (PLP) to form a Schiff base (aldimine), releasing the enzyme’s lysine residue.

  2. Step 2 – Conversion to Pyridoxamine Phosphate (PMP):
    The amino acid donates its amino group to PLP, forming pyridoxamine phosphate (PMP) and releasing a keto acid.

  3. Step 3 – Transfer of Amino Group to Keto Acid:
    PMP then transfers its amino group to another α-keto acid, regenerating PLP and producing a new amino acid.

Overall Reaction:
Amino acid₁ + α-Keto acid₂ ⇌ α-Keto acid₁ + Amino acid₂

Examples:

  • Alanine + α-Ketoglutarate ⇌ Pyruvate + Glutamate (ALT)

  • Aspartate + α-Ketoglutarate ⇌ Oxaloacetate + Glutamate (AST)

Site:
Mainly in the liver, kidney, heart, and muscle

Importance:

  • Links amino acid and carbohydrate metabolism

  • Forms glutamate for urea synthesis

  • Reversible, aiding both synthesis and breakdown of amino acids

Deamination:

Definition:
Deamination is the process by which an amino group (–NH₂) is removed from an amino acid, forming ammonia (NH₃) and a corresponding keto acid.

Types:

  1. Oxidative Deamination: Removal of amino group with oxidation.

    • Enzyme: Glutamate dehydrogenase

    • Reaction: Glutamate + NAD⁺ + H₂O → α-Ketoglutarate + NH₃ + NADH

    • Site: Liver mitochondria

  2. Non-oxidative Deamination: Removal of the amino group without oxidation.

    • Examples: Serine, threonine, and histidine undergo this type.

Importance:

  • Releases free ammonia for urea synthesis.

  • Produces keto acids for energy production, gluconeogenesis, or fatty acid synthesis.

  • Maintains nitrogen balance in the body.

 

1. Formation of Ammonia:
Ammonia is produced in the body from several sources:

  • Oxidative deamination of glutamate (via glutamate dehydrogenase).

  • Deamidation of glutamine and asparagine.

  • Transamination followed by deamination.

  • Bacterial action in the intestine (urease activity on urea).

2. Transport of Ammonia:
Because ammonia is toxic, it is transported in non-toxic forms:

  • As Glutamine:

    • Enzyme: Glutamine synthetase

    • NH₃ + Glutamate → Glutamine (in peripheral tissues)

    • Glutamine travels to the liver or kidney where it is hydrolyzed back to NH₃.

  • As Alanine:

    • Formed in muscle via alanine transaminase (ALT).

    • Alanine carries ammonia to the liver for conversion to urea (Glucose–Alanine cycle).

3. Detoxification and Excretion:

  • In Liver:

    • Ammonia is converted into urea by the urea cycle (main pathway of detoxification).

  • In Kidney:

    • Small amount of ammonia is directly excreted in urine as NH₄⁺ to help maintain acid–base balance.

4. Utilization:

  • Small amounts are used in the synthesis of amino acids, purines, pyrimidines, and glutamine.

5. Toxicity:

  • Excess ammonia causes hyperammonemia, leading to CNS symptoms (confusion, tremor, coma).

  • Normally, blood ammonia is kept below 50 µmol/L by efficient liver function.

 

Formation of Urea

Definition:

  • The urea cycle (also called the ornithine cycle) is the biochemical pathway in the liver by which toxic ammonia (NH₃), produced from amino acid catabolism, is converted into non-toxic urea, which is then excreted by the kidneys.
  • It was discovered by Hans Krebs and Kurt Henseleit in 1932 and represents the first cyclic metabolic pathway identified.

Site of Occurrence:

  • Organ: Liver (hepatocytes)

  • Subcellular Location:

    • Mitochondria: First two reactions

    • Cytosol: Last three reactions

Precursors of Urea:

Urea contains two nitrogen atoms and one carbon atom:

Atom Source
One nitrogen From ammonia (NH₃) produced by oxidative deamination of glutamate
Second nitrogen From aspartate formed by transamination of oxaloacetate
Carbon atom From CO₂ (as bicarbonate, HCO₃⁻)

 

Steps of Urea Formation:

Step 1 – Formation of Carbamoyl Phosphate

  • Enzyme: Carbamoyl phosphate synthetase I (CPS I)

  • Location: Mitochondria

  • Reaction:

    NH3+CO2+2ATP→Carbamoyl phosphate+2ADP+Pi

  • Cofactor: N-Acetylglutamate (NAG) – an essential allosteric activator.

  • Significance: This is the rate-limiting step of the cycle.

Step 2 – Formation of Citrulline

  • Enzyme: Ornithine transcarbamoylase (OTC)

  • Location: Mitochondria

  • Reaction:

    Ornithine+Carbamoyl phosphate→Citrulline+Pi

  • Process: Citrulline is then transported to the cytosol via an ornithine–citrulline antiporter.

Step 3 – Formation of Argininosuccinate

  • Enzyme: Argininosuccinate synthetase

  • Location: Cytosol

  • Reaction:

    Citrulline+Aspartate+ATP→Argininosuccinate+AMP+PPi

  • Significance:

    • Aspartate contributes the second nitrogen of urea.

    • Uses two high-energy phosphate bonds (ATP → AMP).

Step 4 – Cleavage of Argininosuccinate

  • Enzyme: Argininosuccinate lyase (Argininosuccinase)

  • Location: Cytosol

  • Reaction:

    Argininosuccinate→Arginine+Fumarate

  • Significance:

    • Fumarate enters the TCA cycle, forming malate and oxaloacetate (link between the two cycles).

    • This connection is known as the Aspartate–Argininosuccinate shunt.

Step 5 – Formation of Urea and Regeneration of Ornithine

  • Enzyme: Arginase

  • Location: Cytosol

  • Reaction:

    Arginine+H2O→Urea+Ornithine 

  • Significance:

    • Urea is released into blood → transported to kidneys → excreted in urine.

    • Ornithine is recycled back into mitochondria to continue the cycle.

Overall Reaction:

2NH3+CO2+3ATP+H2O→Urea+2ADP+AMP+4Pi+  

  • Energy cost: 3 ATP (4 high-energy bonds) are used for each molecule of urea synthesized.

  • Energy recovery: Oxidation of fumarate via TCA cycle yields ~1 NADH (≈ 3 ATP), partly compensating energy expenditure.

Regulation of Urea Cycle:

  1. Allosteric Regulation:

    • CPS I is activated by N-Acetylglutamate (NAG).

    • Arginine stimulates NAG synthesis → increases urea cycle activity.

  2. Substrate Availability:

    • Increased ammonia or amino acid load enhances urea formation.

  3. Enzyme Induction:

    • High-protein diet or fasting induces the synthesis of urea cycle enzymes.

Physiological Significance:

  • Ammonia detoxification: Converts toxic NH₃ to non-toxic urea.

  • Nitrogen excretion: Main pathway for nitrogen elimination.

  • Interconnection with energy metabolism: Fumarate links to the TCA cycle.

  • Maintenance of acid–base balance: Prevents ammonia accumulation, which raises pH.

Clinical Correlations:

1. Hyperammonemia:

  • Elevated blood ammonia due to defective urea formation.

  • Causes:

    • Liver disease (acquired)

    • Congenital enzyme deficiencies (inherited)

2. Enzyme Deficiencies and Disorders:

Enzyme Deficiency Disorder Key Features
CPS I Hyperammonemia Type I ↑ NH₃, ↓ citrulline
OTC Hyperammonemia Type II ↑ NH₃, ↑ orotic acid (X-linked)
Argininosuccinate synthetase Citrullinemia ↑ Citrulline
Argininosuccinate lyase Argininosuccinic aciduria ↑ Argininosuccinate
Arginase Hyperargininemia ↑ Arginine, neurological symptoms

3. Symptoms:

Vomiting, lethargy, seizures, cerebral edema, coma.

4. Treatment:

  • Dietary: Low-protein diet

  • Drugs: Sodium benzoate, phenylacetate, or phenylbutyrate (bind excess ammonia)

  • Supplement: Arginine or citrulline (depending on deficiency)

  • Severe cases: Liver transplantation

Quantitative Aspect:

  • Daily urea excretion: 25–30 g/day in adults.

  • Constitutes about 80–90% of total urinary nitrogen.

Link with Other Metabolic Pathways:

Cycle/Pathway Connection
TCA Cycle Fumarate from urea cycle enters TCA; CO₂ from TCA used in CPS I reaction.
Amino Acid Metabolism Provides ammonia and aspartate.
Transamination Reactions Form aspartate and glutamate, key intermediates.

 

Mnemonic for Enzymes (in order):

C – O – A – A – A

    1. C – Carbamoyl phosphate synthetase I

    2. O – Ornithine transcarbamoylase

    3. A – Argininosuccinate synthetase

    4. A – Argininosuccinate lyase

    5. A – Arginase

Glycine Metabolism

Introduction

  • Glycine is the simplest amino acid (NH₂-CH₂-COOH).

  • It is non-essential, glucogenic, and plays key roles in protein synthesis, one-carbon metabolism, and biosynthesis of important biomolecules.

Feature Description
Chemical Formula C₂H₅NO₂
Molecular Weight 75 Da
Nature Non-essential, glucogenic amino acid
Chirality Achiral (only amino acid without optical activity)
Side Chain Hydrogen (–H)
Solubility Highly water-soluble
Special Feature Found abundantly in collagen (every 3rd residue)

 

Sources / Synthesis of Glycine

Glycine can be synthesized endogenously in several ways:

Pathway Enzyme Cofactors Location Significance
Serine → Glycine SHMT PLP, THF Cytosol & mitochondria Major contributor; linked to folate cycle
Threonine → Glycine Threonine aldolase PLP Cytosol Minor physiological source
Choline → Betaine → Glycine Dimethylglycine dehydrogenase FAD Mitochondria Connects methylation cycle
Glyoxylate → Glycine AGT PLP Peroxisomes Defects → Primary hyperoxaluria

 

Cofactors:

  • Pyridoxal phosphate (Vitamin B₆)

  • Tetrahydrofolate (THF) for one-carbon transfer

Catabolism of Glycine

 

Metabolic Roles of Glycine

Function Compound Synthesized Enzyme (if applicable)
Heme synthesis Glycine + Succinyl-CoA → δ-Aminolevulinic acid (ALA) ALA synthase
Creatine synthesis Glycine + Arginine → Guanidinoacetate → Creatine Transamidase
Purine synthesis Donates C₄, C₅, N₇ atoms of purine ring —
Glutathione synthesis Glycine + Cysteine + Glutamate → GSH Glutathione synthetase
Bile salt conjugation Bile acids + Glycine → Glycocholic acid, etc. Bile acid–CoA:amino acid N-acyltransferase
Porphyrin & Heme As above —
One-carbon metabolism Forms CH₂-THF via glycine cleavage system —

 

Regulation of Glycine Metabolism

Regulator Effect on Metabolism Notes
PLP (Vitamin B6) Required for SHMT, transaminases, ALA synthase Deficiency ↓ glycine processing
Folate (THF) Necessary for serine–glycine conversion Folate deficiency disrupts 1-carbon metabolism
GCS Activity Controls glycine degradation Deficiency → NKH
Dietary Protein ↑ glycine levels Protein-rich foods boost supply
Hormones Glucagon ↑ catabolism, Insulin ↑ anabolism Affects amino acid turnover
Peroxisomal enzymes Regulate glyoxylate handling Defects → Hyperoxaluria

 

Clinical Significance

Disorder Enzyme Defect / Cause Key Features
Non-ketotic hyperglycinemia (glycine encephalopathy) Defect in glycine cleavage enzyme complex ↑ Glycine in CSF & plasma → severe neurological symptoms, seizures, mental retardation
Primary hyperoxaluria Defective glyoxylate metabolism (↑ glyoxylate → oxalate) Kidney stones, renal failure
Deficiency of THF or B₆ Impaired glycine metabolism Anemia, reduced one-carbon transfer reactions

 

Laboratory Diagnosis

Test Sample Purpose
Plasma amino acid analysis Blood Detect elevated glycine
CSF amino acid profiling CSF Diagnose NKH
Urine oxalate measurement Urine Diagnose hyperoxaluria
Hippurate measurement Urine Evaluate detoxification function
HPLC / GC-MS Plasma/Urine Accurate quantitative analysis

 

Metabolism of Phenylalanine and Tyrosine

Introduction

  • Phenylalanine (Phe) and Tyrosine (Tyr) are aromatic amino acids derived from the shikimate pathway in plants and obtained in humans from diet.

  • Phenylalanine is an essential amino acid, while tyrosine is non-essential (formed from phenylalanine).

  • Both are glucogenic and ketogenic.

  • They serve as precursors for several vital molecules — catecholamines (dopamine, norepinephrine, epinephrine), thyroid hormones, and melanin.

Conversion of Phenylalanine to Tyrosine:

Component Description
Enzyme Phenylalanine hydroxylase (PAH)
Location Liver
Cofactor BH₄, Fe²⁺
Importance Prevents toxic buildup of phenylalanine
Defects Cause Phenylketonuria (PKU)

Reaction:Phenylalanine+O2+Tetrahydrobiopterin(BH4)→Tyrosine+H2O+Dihydrobiopterin(BH2)

Enzyme: Phenylalanine hydroxylase

Cofactors:

  • Tetrahydrobiopterin (BH₄) – acts as a reducing cofactor
  • Fe²⁺ (Iron) – required for enzyme activity
  • Oxygen (O₂) – provides one atom for hydroxylation

Location: Liver cytosol

Mechanism:

  • The enzyme adds a hydroxyl group (–OH) to the para position of the benzene ring of phenylalanine.
  • This converts phenylalanine into tyrosine, making it hydroxylated at the 4th position (p-hydroxyphenylalanine).
  • During the reaction, BH₄ is oxidized to BH₂ and later regenerated by dihydropteridine reductase using NADPH.

Significance:

  • This is the first step in phenylalanine catabolism.
  • It converts the essential amino acid (phenylalanine) into a non-essential amino acid (tyrosine).
  • Tyrosine then serves as a precursor for melanin, catecholamines (dopamine, epinephrine, norepinephrine), and thyroid hormones.

Clinical Importance:

  • Deficiency of phenylalanine hydroxylase or BH₄ causes Phenylketonuria (PKU) → accumulation of phenylalanine and its toxic metabolites leading to mental retardation and hypopigmentation.

 

Regulation of Phenylalanine and Tyrosine Metabolism

Regulation Type Molecules Involved Effect
Feedback inhibition High tyrosine Inhibits phenylalanine hydroxylase
Hormonal Glucocorticoids Induce tyrosine aminotransferase
Cofactor availability BH₄, PLP, Vit C Controls specific enzymes
Genetic PAH, HGD, FAH mutations Cause metabolic disorders

 

Metabolic Disorders of Phenylalanine and Tyrosine

Phenylketonuria (PKU) is an inborn error of phenylalanine metabolism associated with the inability to convert phenylalanine to tyrosine. Ratio 1 in 20,000 newborns.

Types Condition  Enzyme defects
Type 1 Classical type of PKU Phenylalanine hydroxylase enzyme deficiency
Type 2  Persistent hyperphenylalaninaemia Phenylalanine hydroxylase enzyme deficiency
Type 3 Transient mild hyperphenylalaninaemia Phenylalanine hydroxylase enzyme is delayed 
Type 4 Dihydropterine reductase deficiency Dihydropterine deficiency
Type 5 Abnormal Dihydropterine function Dihydropterine synthesis defects

 

 

 

 

 

Tyrosinemia

There are three types of tyrosinemia:

  1. Tyrosinemia type-I (Tyrosinosis/Hepatorenal tyrosinemia)
  2. Tyrosinemia type-II (Richner-Hanhart syndrome)
  3. Neonatal tyrosinemia

Alkaptonuria

Definition: Alkaptonuria is a rare autosomal recessive metabolic disorder characterized by the accumulation of homogentisic acid due to a deficiency in the enzyme homogentisate oxidase.

Enzyme Defect

  • The defective enzyme in alkaptonuria is homogentisate oxidase in tyrosine metabolism.
  • Homogentisate accumulates in tissues and blood and is excreted into urine. The urine of alkaptonuria patients resembles coke in colour.

Biochemical Manifestations

  1. Homogentisic Acid Accumulation:
    • The main biochemical defect is the elevated levels of homogentisic acid in the body.
    • This compound is toxic and can lead to various pathological effects.
  2. Urine Color Change:
    • Urine from affected individuals darkens upon exposure to air due to the oxidation of homogentisic acid. This can happen within a few hours and is a hallmark feature of the disease.
    • Freshly voided urine may appear normal but darkens rapidly when left standing.
  3. Ochronosis:
    • Chronic accumulation of homogentisic acid can lead to tissue deposits in connective tissues, known as ochronosis.
    • Common sites include the cartilage of joints, intervertebral discs, and the skin. This can cause discolouration and degenerative joint disease.
  4. Systemic Effects:
    • Patients may experience early-onset arthritis, especially in large joints (e.g., hips, knees).
    • Other complications include potential heart valve issues and kidney stones.

Diagnosis

  1. Clinical Evaluation:
    • Diagnosis often starts with a clinical suspicion based on symptoms such as dark urine and joint pain.
  2. Urine Analysis:
    • Colour Test: The darkening of urine upon standing is a critical diagnostic sign.
    • Chemical Analysis: Urine can be tested for the presence of homogentisic acid using qualitative and quantitative methods, such as:
      • HPLC (High-Performance Liquid Chromatography): Measures the levels of homogentisic acid.
      • Spot Tests: A simple qualitative test where a few drops of urine can react with specific reagents to indicate the presence of homogentisic acid.
  1. Genetic Testing:
    • Identification of mutations in the HGD gene can confirm the diagnosis.
    • Genetic counselling may be recommended for affected individuals and their families.

Management

While there is currently no cure for alkaptonuria, management focuses on symptomatic relief and preventing complications:

  1. Lifestyle Modifications:
    • Encourage a balanced diet with limited intake of phenylalanine and tyrosine, although dietary restrictions may vary in severity based on individual cases.
    • Maintain hydration to help reduce the risk of kidney stones.
  2. Pain Management:
    • Nonsteroidal anti-inflammatory drugs (NSAIDs) can be used to manage joint pain.
    • In severe cases, physical therapy or joint replacement surgery may be necessary.
  3. Monitoring:
    • Regular followup to monitor joint health and function.
    • Periodic assessment of urine for homogentisic acid levels can help gauge the condition’s progression.
  4. Research and Experimental Therapies:
    • Ongoing research explores potential treatments, including enzyme replacement therapy and dietary supplements, but these are not yet standard practice.

Phenylketonuria

Phenylketonuria (PKU) is a genetic metabolic disorder caused by a defect in the enzyme phenylalanine hydroxylase (PAH). This condition affects the body’s ability to metabolize the amino acid phenylalanine, leading to various biochemical and clinical manifestations.

Enzyme Defect

  • Enzyme: Phenylalanine hydroxylase (PAH)
  • Function: PAH catalyzes the conversion of phenylalanine to tyrosine, another amino acid.
  • Deficiency: When PAH is deficient or absent, phenylalanine accumulates in the body, leading to toxic effects, particularly in the brain. The incidence of PKU is 1 in 10,000 births.

Biochemical Manifestations

  1. Elevated Phenylalanine Levels:
    • The hallmark of PKU is significantly increased levels of phenylalanine in the blood (hyperphenylalaninemia).
    • Normal phenylalanine levels are usually between 0.5 to 1.5 mg/dL, while levels in untreated PKU can exceed 20 mg/dL.
  2. Deficiency of Tyrosine:
    • Since PAH converts phenylalanine to tyrosine, its deficiency leads to reduced levels of tyrosine, which is essential for neurotransmitter synthesis (dopamine, norepinephrine).
  3. Metabolite Accumulation:
    • Increased phenylalanine can be converted to phenylpyruvate, which is then excreted in urine, along with other phenylalanine derivatives.
  4. Neurological Effects:
    • High levels of phenylalanine are neurotoxic, leading to developmental delays, intellectual disability, seizures, and behavioural problems if untreated.
  5. Other Symptoms:
    • Patients may develop lighter skin and hair due to reduced melanin production (tyrosine is a precursor for melanin).

Diagnosis

  1. Newborn screening:
    • PKU is typically diagnosed through routine newborn screening programs that measure blood phenylalanine levels.
    • A heel prick test is performed shortly after birth, usually within the first week.
  2. Blood Tests:
    • Phenylalanine Levels: A blood sample is analyzed for elevated levels of phenylalanine. A level above the threshold indicates a risk for PKU.
    • Tandem Mass Spectrometry: This advanced technique can confirm elevated phenylalanine and is often used in newborn screening.
  3. Genetic Testing:
    • Confirmatory testing can involve genetic analysis to identify mutations in the PAH gene, confirming the diagnosis and subtype of PKU.
    • This testing can also help assess the risk for family members.
  4. Clinical Evaluation:
    • If PKU is suspected, a thorough clinical assessment will be conducted, looking for signs of neurological impairment or developmental delays.

Management

  1. Dietary Management:
    • The cornerstone of PKU management is a strict, lifelong low-phenylalanine diet.
    • Patients avoid high-protein foods (meat, fish, eggs, dairy, nuts) and certain grains.
    • Special medical formulas that provide essential nutrients without phenylalanine are often used.
  2. Monitoring:
    • Monitoring blood phenylalanine levels is crucial to ensure they remain within target ranges, typically below 6 mg/dL.
    • Dietary adjustments may be necessary based on these levels.
  3. Supplementation:
    • Tyrosine supplementation may be necessary due to its reduced levels in PKU patients.
  4. Emerging Therapies:
    • New treatments, such as enzyme replacement therapy, pharmacological therapies (e.g., sapropterin dihydrochloride), and gene therapy, are being researched and may offer additional options.
  5. Support Services:
    • Nutritional counselling and support groups can provide essential education and emotional support for families managing the condition.

Maple Syrup Urine Disease

Maple Syrup Urine Disease (MSUD) is a rare genetic metabolic disorder caused by a defect in the branched-chain alpha-keto acid dehydrogenase (BCKAD) complex, which is essential for the metabolism of branched-chain amino acids (BCAAs): leucine, isoleucine, and valine.

Enzyme Defect

  • Enzyme Complex: Branched-chain alpha-keto acid dehydrogenase (BCKAD) Complex
  • Gene Mutations: Mutations can occur in several genes that encode components of the BCKAD complex, including:
    • BCKDHA (alpha component)
    • BCKDHB (beta component)
    • DBT (dihydrolipoamide branched-chain transacylase)
  • Function: The BCKAD complex catalyzes the oxidative decarboxylation of branched-chain alpha-keto acids derived from the BCAAs.
  • Deficiency: When this complex is deficient, branched-chain amino acids accumulate and their corresponding alpha-keto acids in the blood and urine.

Biochemical Manifestations

  1. Elevated Branched-Chain Amino Acids:
    • Blood levels of leucine, isoleucine, and valine become significantly elevated. Normal levels are typically below 150 μmol/L for leucine, 40 μmol/L for isoleucine, and 100 μmol/L for valine.
    • In untreated MSUD, leucine levels can exceed 1,000 μmol/L.
  2. Accumulation of Alpha-Keto Acids:
    • Alongside elevated BCAAs, their corresponding alpha-keto acids (such as alpha-ketoisocaproic acid) accumulate, which can be toxic, especially to the nervous system.
  3. Neurological Symptoms:
    • Toxic levels of BCAAs and their metabolites can lead to neurological issues, including lethargy, seizures, and developmental delays.
  4. Urine Characteristics:
    • The condition is named for the sweet, maple syrup-like odour of the urine due to the presence of branched-chain keto acids.

Diagnosis

  1. Newborn Screening:
    • MSUD is typically diagnosed through routine newborn screening programs, which test for elevated levels of leucine and other BCAAs in dried blood spots collected shortly after birth.
  2. Clinical Presentation:
    • Symptoms often appear within the first few days of life, including poor feeding, vomiting, lethargy, and irritability.
  3. Blood Tests:
    • Confirmatory blood tests measure the levels of branched-chain amino acids, showing significant elevations of leucine, isoleucine, and valine.
  4. Urine Analysis:
    • Urinalysis may reveal the presence of branched-chain keto acids, which specific chemical tests can detect.
  5. Genetic Testing:
    • Genetic testing can confirm the diagnosis by identifying mutations in the genes associated with the BCKAD complex.
    • This testing can also help determine the specific subtype of MSUD, as there are several variants (classic, intermediate, and thiamine-responsive).

Management

  1. Dietary Management:
    • The primary treatment for MSUD involves a strict diet low in branched-chain amino acids, particularly leucine.
    • Special medical formulas that provide essential amino acids without BCAAs are essential for growth and development.
  2. Monitoring:
    • Regularly monitoring blood amino acid levels is crucial to prevent toxic accumulation and adjust dietary intake as needed.
  3. Emergency Protocols:
    • In times of illness or stress, rapid intervention may be required to manage acute metabolic crises, which can be life-threatening. This often involves hospitalization and intravenous fluids.
  4. Potential Therapies:
    • Research is ongoing into new treatments, including enzyme replacement therapy, gene therapy, and alternative dietary strategies.
  5. Support Services:
    • Nutritional counselling and family support resources are vital for managing the condition effectively.

Albinism

Albinism is a group of genetic disorders characterized by a deficiency or absence of melanin production in the skin, hair, and eyes. The condition arises from defects in specific enzymes involved in the melanin biosynthesis pathway.

Enzyme Defect

  1. Common Enzyme Defects:
    • Tyrosinase: The most common defect occurs in tyrosinase, which catalyzes the conversion of tyrosine to DOPA (dihydroxyphenylalanine) and then to dopaquinone, a melanin precursor. This is associated with Oculocutaneous Albinism Type 1 (OCA1).
    • Other Enzymes: Defects in other enzymes like tyrosinase-related protein 1 (TYRP1) and Dopachrome tautomerase (DCT) lead to other forms of albinism.
  2. Gene Mutations:
    • TYR (tyrosinase gene), OCA2 (associated with OCA2), and TYRP1 genes are among the most frequently mutated genes in different types of albinism.

Biochemical Manifestations

  1. Reduced Melanin Production:
    • Affected individuals have significantly reduced or absent melanin levels in the skin, hair, and eyes.
    • The lack of melanin leads to lighter pigmentation and can result in white or light-coloured hair and skin.
  2. Ocular Abnormalities:

Common ocular manifestations include:

    • Nystagmus: Involuntary eye movements.
    • Strabismus: Misalignment of the eyes.
    • Photophobia: Sensitivity to bright light.
    • Reduced Visual Acuity: Impaired vision due to improper retina development.

3. Increased Sun Sensitivity:

    • Individuals with albinism are more susceptible to sunburn and skin damage due to the lack of protective melanin.
    • They have a higher risk of developing skin cancers, including melanoma.

Diagnosis

  1. Clinical Evaluation:
    • Diagnosis often begins with a clinical examination that reveals characteristic features such as light skin, hair, eye colour, and ocular abnormalities.
  2. Family History:
    • A family history of albinism can support the diagnosis, as many forms are inherited in an autosomal recessive manner.
  3. Genetic Testing:
    • Molecular genetic testing can confirm the diagnosis by identifying mutations in the relevant genes.
    • This testing can also help determine the specific type of albinism.
  4. Ophthalmologic Examination:
    • A detailed eye examination can reveal specific ocular defects associated with albinism, such as foveal hypoplasia (underdevelopment of the fovea) and abnormal retinal structure.
  5. Skin Biopsy:
    • Sometimes, a skin biopsy may be performed to assess melanin production and distribution.

Management

  1. Sun Protection:
    • Individuals with albinism should take strict measures to protect their skin from UV exposure, including high-SPF sunscreen, protective clothing, and sunglasses.
  2. Vision Support:
    • Visual aids and corrective lenses may be necessary to improve visual acuity.
    • Regular eye exams are essential for monitoring and addressing ocular issues.
  3. Educational Support:
    • Specialized educational resources may be required to accommodate visual impairments.
  4. Psychosocial Support:
    • Counselling and support groups can help individuals and families cope with the challenges associated with living with albinism, including social stigma and psychological impacts.

Hartnup disorder

Hartnup disorder is a rare genetic condition characterized by the impaired absorption of certain amino acids, primarily neutral ones, in the kidneys and intestines. A defect in a specific transporter protein causes this disorder.

Enzyme Defect

  • Transporter Defect: The primary defect in Hartnup disorder is in the gene, which encodes a sodium-dependent neutral amino acid transporter.
  • Affected Transport: This transporter reabsorbs neutral amino acids (such as tryptophan, leucine, isoleucine, and phenylalanine) in the renal tubules and the intestines.

Biochemical Manifestations

  1. Amino Aciduria:
    • Due to the defective transporter, neutral amino acids are not effectively reabsorbed, leading to excessive urine loss (aminoaciduria).
    • This results in low blood levels of these amino acids (hypoaminoacidemia).
  2. Tryptophan Deficiency:
    • The loss of tryptophan can lead to decreased serotonin and niacin (vitamin B3) synthesis, as tryptophan is a precursor for both.
    • Niacin deficiency can result in symptoms similar to pellagra, including diarrhoea, dermatitis, and dementia.
  3. Neurological Symptoms:
    • Some individuals may experience neurological issues due to low levels of neurotransmitters (e.g., serotonin) derived from tryptophan. Symptoms can include ataxia, psychiatric disturbances, and mood changes.
  4. Skin Manifestations:
    • Some patients may develop photosensitivity and skin rashes, especially when exposed to sunlight, due to the effects of tryptophan deficiency and niacin deficiency.

Diagnosis

  1. Clinical Evaluation:
    • Diagnosis often begins with a clinical assessment of photosensitivity, ataxia, and neurological manifestations.
    • Family history may provide additional context, as Hartnup disorder is inherited in an autosomal recessive manner.
  2. Urine Analysis:
    • A 24-hour urine collection can reveal elevated levels of neutral amino acids, particularly tryptophan, leucine, and isoleucine.
  3. Blood Tests:
    • Blood tests may show low levels of neutral amino acids, especially tryptophan.
  4. Genetic Testing:
    • Molecular genetic testing can confirm the diagnosis by identifying mutations in the SLC6A19
    • Genetic counselling may be recommended for affected individuals and their families.
  5. Response to Niacin Supplementation:
    • Sometimes, a trial of niacin supplementation can help assess the impact of the deficiency on symptoms and provide supportive evidence for the diagnosis.

Management

  1. Dietary Management:
    • A balanced diet rich in proteins and potentially supplemented with essential amino acids may help manage symptoms.
    • Some individuals may benefit from a niacin-rich diet or supplementation to prevent deficiency.
  2. Symptomatic Treatment:
    • Addressing specific symptoms, such as skin rashes or neurological issues, may require additional treatment and support.
  3. Sun Protection:
    • Patients may need to avoid excessive sun exposure and use sunscreen to manage photosensitivity.
  4. Regular Monitoring:
    • Regular follow-ups with healthcare providers are important to monitor amino acid levels and overall health.

MCQs

1. The first step in dietary protein digestion begins in the:

A. Mouth
B. Stomach
C. Duodenum
D. Ileum

2. The major proteolytic enzyme of the stomach is:

A. Trypsin
B. Pepsin
C. Chymotrypsin
D. Elastase

3. Pepsinogen is activated to pepsin by:

A. Trypsin
B. HCl
C. Secretin
D. Bicarbonate

4. Enteropeptidase converts:

A. Trypsin to trypsinogen
B. Trypsinogen to trypsin
C. Pepsinogen to pepsin
D. Proelastase to elastase

5. The major site of amino acid absorption is:

A. Stomach
B. Duodenum
C. Jejunum
D. Colon

6. Amino acids are absorbed by:

A. Primary active transport
B. Secondary active transport
C. Diffusion
D. Facilitated diffusion

7. Transamination requires which coenzyme?

A. NAD⁺
B. PLP (Vitamin B6)
C. Biotin
D. THF

8. The major enzyme for removing the amino group from glutamate is:

A. ALT
B. AST
C. Glutamate dehydrogenase
D. Transaminase

9. Glutamate dehydrogenase uses which cofactors?

A. NAD⁺ or NADP⁺
B. FAD
C. THF
D. PLP

10. Urea cycle occurs primarily in the:

A. Brain
B. Kidney
C. Liver
D. Intestine

11. The first amino acid used in urea cycle is:

A. Glycine
B. Glutamine
C. Arginine
D. Ammonia

12. Carbamoyl phosphate synthase I is located in:

A. Cytosol
B. Mitochondria
C. ER
D. Nucleus

13. The rate-limiting enzyme of urea cycle is:

A. Arginase
B. CPS-I
C. ASS
D. ASL

14. CPS-I requires which activator?

A. Glutamate
B. Aspartate
C. N-Acetylglutamate
D. Fumarate

15. Ornithine transcarbamylase (OTC) deficiency leads to:

A. Hyperglycinemia
B. Hyperammonemia
C. Maple syrup urine disease
D. Hartnup disease

16. In the liver, ammonia is converted to:

A. Uric acid
B. Creatinine
C. Urea
D. Glucose

17. During prolonged fasting, major gluconeogenic amino acid is:

A. Phenylalanine
B. Leucine
C. Alanine
D. Tryptophan

18. Glucogenic amino acids produce:

A. Acetoacetate
B. Acetyl-CoA
C. TCA cycle intermediates
D. Fatty acids

19. Ketogenic amino acids include:

A. Leucine & Lysine
B. Alanine & Glycine
C. Valine & Proline
D. Histidine & Arginine

20. Which amino acid forms serotonin?

A. Tyrosine
B. Tryptophan
C. Phenylalanine
D. Histidine

21. Phenylalanine hydroxylase requires:

A. Biotin
B. THF
C. BH4
D. PLP

22. Transamination of alanine produces:

A. Acetyl-CoA
B. Pyruvate
C. Oxaloacetate
D. α-Ketoglutarate

23. Glutamine serves as a major carrier of:

A. Hydrogen ions
B. CO₂
C. Ammonia
D. Uric acid

24. Cystinuria is caused by defective transport of:

A. Neutral amino acids
B. Acidic amino acids
C. Basic amino acids & cystine
D. Aromatic amino acids

25. Maple syrup urine disease involves defect in metabolism of:

A. Aromatic amino acids
B. Sulfur-containing amino acids
C. Branched-chain amino acids
D. Acidic amino acids

26. Alkaptonuria is due to defect in:

A. Phenylalanine hydroxylase
B. Homogentisate oxidase
C. Tyrosinase
D. DOPA decarboxylase

27. Phenylketonuria results from deficiency of:

A. BH2
B. CPS-I
C. Phenylalanine hydroxylase
D. Tryptophan hydroxylase

28. Carbamoyl phosphate is formed from:

A. CO₂ + NH₃ + ATP
B. CO₂ + H₂O + ATP
C. Glutamine + ATP
D. Urea + ATP

29. The step in urea cycle that releases urea is catalyzed by:

A. ASS
B. ASL
C. CPS-I
D. Arginase

30. Fumarate formed in urea cycle enters:

A. Glycolysis
B. TCA cycle
C. PPP
D. FA synthesis

31. Nitrogen balance is positive in:

A. Illness
B. Fasting
C. Growth & pregnancy
D. Burns

32. Kwashiorkor is characterized by:

A. Edema
B. Muscle wasting only
C. No fatty liver
D. Low insulin

33. Marasmus is characterized by:

A. Edema
B. Severe wasting
C. Fatty liver
D. Hypoalbuminemia only

34. During starvation, muscle releases:

A. Leucine and lysine
B. Alanine and glutamine
C. Phenylalanine and tyrosine
D. Methionine and cysteine

35. The major amino acid for ammonium trapping in the kidney is:

A. Alanine
B. Glycine
C. Glutamine
D. Serine

36. Essential amino acids are:

A. Alanine, glycine, serine
B. Leucine, valine, lysine
C. Tyrosine, cysteine
D. Proline, arginine in adults

37. Tyrosine is synthesized from:

A. Leucine
B. Glycine
C. Phenylalanine
D. Valine

38. Dopa decarboxylase requires:

A. PLP
B. THF
C. Biotin
D. FAD

39. Amino acids important for one-carbon metabolism include:

A. Glycine & serine
B. Leucine & tryptophan
C. Tyrosine & phenylalanine
D. Arginine & lysine

40. Homocysteine is formed from:

A. Serine
B. Methionine
C. Lysine
D. Glycine

41. SAM (S-adenosylmethionine) is:

A. Methyl donor
B. Biotin carrier
C. Antioxidant
D. Precursor of urea

42. Creatine is synthesized from:

A. Glycine + Arginine
B. Glycine + Lysine
C. Methionine + Tyrosine
D. Alanine + Glutamine

43. Major amino acid in collagen is:

A. Valine
B. Serine
C. Glycine
D. Glutamate

44. Hydroxylation of proline requires:

A. Vit B6
B. Vit C
C. Vit K
D. FAD

45. Nitric oxide is formed from:

A. Glycine
B. Arginine
C. Proline
D. Serine

46. GABA is synthesized from:

A. Tyrosine
B. Serine
C. Glutamate
D. Glycine

47. Which amino acid is purely ketogenic?

A. Isoleucine
B. Phenylalanine
C. Tyrosine
D. Leucine

48. Glucose-alanine cycle transfers:

A. CO₂
B. Ammonia to liver
C. Ketones
D. Fatty acids

49. Which amino acid forms histamine?

A. Histidine
B. Arginine
C. Tryptophan
D. Tyrosine

50. Rate of protein turnover is highest in:

A. Bone
B. Skin
C. Intestinal mucosa
D. Muscle

✅ ANSWER KEY

1-B
2-B
3-B
4-B
5-C
6-B
7-B
8-C
9-A
10-C
11-D
12-B
13-B
14-C
15-B
16-C
17-C
18-C
19-A
20-B
21-C
22-B
23-C
24-C
25-C
26-B
27-C
28-A
29-D
30-B
31-C
32-A
33-B
34-B
35-C
36-B
37-C
38-A
39-A
40-B
41-A
42-A
43-C
44-B
45-B
46-C
47-D
48-B
49-A
50-C