Protein Metabolism

• Protein digestion and absorption are critical processes that enable the body to utilize dietary proteins for various physiological functions, including tissue repair, enzyme synthesis, and hormone production.
• The process involves multiple anatomical structures and enzymatic actions.

Digestion of Protein

Digestion and absorption of proteins

• Proteolytic enzymes break down dietary proteins into their constituent amino acids.
• These enzymes are produced by:

  Stomach 

  Pancreas

  Small intestine.

• There is no digestion of protein in the mouth.

Gastric Enzymes

1. Pepsin

• • Source: Secreted by gastric chief cells in the stomach as an inactive precursor called pepsinogen.
  • Activation: Pepsinogen is activated to pepsin by the acidic environment (pH 1.5 to 3.5) created by hydrochloric acid (HCl).
  • Function: Pepsin cleaves peptide bonds, primarily adjacent to aromatic amino acids (phenylalanine, tryptophan, and tyrosine), breaking down proteins into smaller polypeptides.

2. Pancreatic Enzymes

Once chyme enters the small intestine, pancreatic enzymes are released to continue protein digestion.

Trypsin

• • Source: Secreted as trypsinogen by the pancreas.
  • Activation: The enzyme enteropeptidase (or enterokinase) activates in the small intestine.
  • Function: Trypsin cleaves peptide bonds on the carboxyl side of lysine and arginine residues, producing smaller peptides.

Chymotrypsin

• • Source: Secreted as chymotrypsinogen by the pancreas.
  • Activation: Activated by trypsin.
  • Function: Chymotrypsin preferentially cleaves peptide bonds adjacent to aromatic amino acids, further breaking down peptides.

Carboxypeptidase

• • Source: Secreted as procarboxypeptidase by the pancreas.
  • Activation: Activated by trypsin.
  • Function: Carboxypeptidase removes terminal amino acids from the carboxyl end of peptides, producing free amino acids.

3. Brush Border Enzymes

Located on the microvilli of enterocytes in the small intestine, these enzymes finalize protein digestion.

Aminopeptidases

• • Function: Cleave off the terminal amino acids from the amino end of peptides, converting them into free amino acids.

Dipeptidases

• • Function: Hydrolyze dipeptides into free amino acids, completing the digestion of peptides into absorbable units.

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Absorption of Proteins

Mechanisms of Absorption

1  Transport Mechanisms:

• • Sodium-Dependent Active Transport: Free amino acids are absorbed via specific transporters that require sodium co-transport, utilizing the sodium gradient established by the Na⁺/K⁺ ATPase pump.
  • Facilitated Diffusion: Small peptides (di- and tri-peptides) can also be absorbed through facilitated diffusion mechanisms.

2. Enterocyte Processing

• • Intracellular Peptide Hydrolysis: Informed di- and tri-peptides within the enterocytes can be hydrolyzed into free amino acids by cytosolic peptidases.
  • Release into Circulation: Free amino acids are transported across the basolateral membrane into the portal circulation, entering the bloodstream.

 

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Formation of Urea

• • Urea is the end product of protein metabolism. The nitrogen of amino acids removed in the form of ammonia is detoxified by converting it to urea.
  • The formation of urea by “Kreb’s Henseleit urea cycle” is the ultimate route for the metabolic disposal of ammonia.
  • Urea is produced exclusively by the liver and then is transported through blood to the kidneys for excretion in the urine.
  • Urea is formed from ammonia, carbon dioxide and α-amino nitrogen of aspartate, which requires ATP.
  • Enzymes catalyzing the urea cycle reactions are distributed between the mitochondria and the cytosol of the liver.

 

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Urea Cycle

Energy Cost of Urea Cycle

Four ATPs are consumed in the synthesis of each molecule of urea as follows:

• Two ATP are needed to make carbamoyl phosphate.
  • One ATP serves as a source of phosphate
  • Second, ATP is converted to AMP + PPi.

• One ATP is required to make arginosuccinate.
• One ATP is required to restore AMP to ATP.

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Disorders of the Urea Cycle

Disruptions in any of the enzymes involved in the urea cycle can lead to urea cycle disorders (UCDs), resulting in the accumulation of ammonia in the blood, which is toxic and can cause severe neurological damage. Here are some key disorders:

1. Ornithine Transcarbamylase Deficiency (OTC)

• • Cause: Deficiency of OTC enzyme.
  • Symptoms: hyperammonaemia, vomiting, lethargy, seizures.
  • Inheritance: X-linked (more common in males).

2. Citrullinemia

• • Cause: Deficiency of arginosuccinate synthetase (ASS).
  • Symptoms: Hyperammonaemia, developmental delay, neurological deficits.
  • Inheritance: Autosomal recessive.

3. Argininosuccinic Aciduria

• • Cause: Deficiency of argininosuccinate lyase (ASL).
  • Symptoms: Hyperammonemia, developmental delay, periodic crises of hyperammonemia.
  • Inheritance: Autosomal recessive.

4. Argininemia

• • Cause: Deficiency of arginase (ARG).
  • Symptoms: Moderate hyperammonemia, spasticity, developmental delay.
  • Inheritance: Autosomal recessive.

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Metabolism of Phenylalanine and Tyrosine

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 hyper phenylalaninaemia	Phenylalanine hydroxylase enzyme deficiency
Type 3	Transient mild hyperphenylalaninaemia	Phenylalanine hydroxylase enzyme 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

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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.

3. 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.

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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.

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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.

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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.

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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.