Protein Chemistry

Proteins are the high molecular weight mixed polymers of α-amino acids joined with peptide linkage (-CO-N H-). Proteins are the chief constituents of all living matter. They contain carbon, hydrogen, nitro­gen and sulphur, and some contain phosphorus.

Classification of Proteins

 

Classification of proteins is done based on the following:

• Shape
• Constitution
• Nature of molecules

Based on shape

• Fibrous protein 

We can find these proteins in animals, which are insoluble in water. Fibrous proteins resist proteolytic enzymes, are coiled, and exist in threadlike structures to form fibres. e.g. collagen, actin, myosin, keratin in hair, claws, feathers, etc.

• Globular proteins

These proteins, unlike fibrous proteins, are soluble in water. They are made up of polypeptides that are coiled about themselves to form oval or spherical molecules, e.g. albumin, insulin, and hormones like oxytocin etc.

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Based on Constitution

1. Simple proteins

• Albumins: Soluble in water, coagulable by heat and precipitated at high salt concentrations.

Examples – The Serum albumin, egg albumin, lactalbumin (Milk), leucosis (wheat), and legumelin (soybeans)

• Globulins: Insoluble in water, soluble in dilute salt solutions and precipitated by half 1 saturated salt solution.

Examples – Serum globulin, vitellin (egg yolk), tuberin (potato), myosinogen (muscle), and legumin (peas).

• Glutelins: Insoluble in water but soluble in dilute acids and alkalis. Mostly found in plants.

Examples – Glutenin (wheat) and oryzenin (rice).

• Prolamines: Insoluble in water and absolute alcohol but soluble in 70 to 80 per cent alcohol.

Examples – Gliadin (wheat) and zein (maize).

• Protamines: Basic proteins of low molecular weight. Soluble in water, dilute acids and alkaline.

Examples – Salmine (salmon sperm).

• Histones: Soluble in water and insoluble in very dilute ammonium hydroxide.

Examples – Globin of hemoglobin and thymus histones.

• Scleroproteins: Insoluble in water, dilute acids and alkalis.

Examples – Keratin (hair, horn, nail, hoof and feathers), collagen (bone, skin), and elastin (ligament).

 

2. Conjugated Proteins

• Nucleoproteins: Composed of simple basic proteins (Pro­tamines or histones) with nucleic acids in nuclei. Soluble in water.

Examples – Nucleoprotamines and Nucleohistones.

• Lipoproteins: Combination of proteins with lipids, such ‘as fatty acids, cholesterol phospholipids, etc.

Examples – The Lipoproteins of egg yolk, milk and cell membranes, and blood lipoproteins.

• Glycoproteins: Combination of proteins with carbohydrates (mucopolysaccharides).

Examples – Mucin (saliva), Ovomucoid (egg white), Osseomucoid (bone), and Tendomucoid (tendon).

• Phosphoproteins: Contains phosphorus radicals.

Examples – Caseinogen (milk) and ovovitellin (egg yolk).

• Metalloproteins: Contain metal ions

Examples – Siderophilin (Fe) and ceruloplasmin (Cu).

• Chromoproteins: They contain porphyrin (with a metal ion)

Examples – Haemoglobin, myoglobin, catalase, peroxidase, cytochromes.

• Flavoproteins: They contain riboflavin.

Examples – Flavoproteins of liver and kidney.

 

3. Derived Protein

Primary derivatives

• Proteins: Derived in the early stage of protein hydrolysis by dilute acids, enzymes or alkalis.

Examples – Fibrin from fibrinogen.

• Metaproteins: Derived in the later stage of protein hydrolysis by slightly stronger acids and alkalis.

Examples – Acid and alkali metaproteins.

• Coagulated proteins: They are denatured proteins formed by the action of heat. X-rays, ultraviolet rays, etc.

Examples are cooked proteins and coagulated albumins.

Secondary derived proteins

They are the degraded product of protein formed due to the breakdown of the peptide bond.

• • Proteoses
  • Peptones

Based on the nature of Molecules

• Acidic proteins: They exist as anions and contain acidic amino acids. e.g. blood groups.
• Basic proteins: They exist as cations and are rich in basic amino acids, e.g. lysine, arginine, etc.

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Structure of proteins

Proteins exhibit four levels of organization

1. Primary structure

• The sequence of amino acids forming the backbone of proteins and the location of any disulfide bond in a protein is called the primary structure of the protein
• In proteins, amino acids are joined covalently by peptide bonds, formed between the α-carboxyl group of one amino acid and the α-amino group of another with the elimination of a water molecule.
• Linkage of many amino acids through peptide bonds results in an unbranched polypeptide chain.

2. Secondary structure

Primary structure hydrogen bonding between the hydrogen of NH and oxygen of C=O groups of the polypeptide chain occurs, which gives rise to the folding or twisting of the primary structure.

Thus, regular folding and twisting of the polypeptide chain brought about by hydrogen bonding is called the secondary structure of the protein. The most important kinds of secondary structure are:

– α-Helix (helicoidal structure)

– β-Pleated sheet (stretched structure)

α-Helix

• It is called α because it was the first structure elucidated by Pauling and Corey. If a backbone of polypeptide chains is twisted by an equal amount of each α-carbon, it forms a coil or helix. The helix is a rod-like structure.
• Hydrogen bonds stabilize the helix between the NH and CO groups of the same chain.
• The axial distance between adjacent amino acids is 1.5 Å and gives 3.6 amino acid residues per turn of helix.

β-Pleated Sheet Structure or Stretched State Structure

• Pauling and Corey discovered another type of structure, which they named β-pleated sheet.
• The surfaces of β-sheet appear “pleated”, and these structures are therefore often called “β-pleated sheets’.
• A polypeptide chain in the β-pleated sheet is almost fully extended rather than tightly coiled as in the α-helix.
• α-helix, β-pleated sheets are composed of two or more polypeptide chains.
• β-pleated sheet is stabilized by hydrogen bonds between NH and C=O groups in a different polypeptide chain.

Polypeptide chains in β-pleated sheet conformation can occur in two ways:

1. Parallel pleated sheet
2. Anti-parallel pleated sheet

1. Parallel pleated sheet

The polypeptide chains lie side-by-side and in the same direction so that their N-terminal residues are at the same end

2. Parallel pleated sheet

The polypeptide chains lie in opposite directions, i.e. N-terminal end of one is next to the C-terminal of the other. It is stabilized by interchain hydrogen bonding.

3. Tertiary structure

• With its secondary structure, the peptide chain may be further folded and twisted about itself, forming a three-dimensional arrangement of the polypeptide chain.
• Amino acid residues that are very distant from one another in the sequence can be brought very near due to the folding and thus form regions essential for the functioning of the protein, like the active site or catalytic site of enzymes.

Tertiary Structure Stabilizing Forces

• Hydrogen bonds
• Hydrophobic interactions
• Van der Waals forces
• Disulfide bond
• Ionic (electrostatic) bonds or salt bridges.

 

4. Quaternary structure

More than one polypeptide chain (polymeric) has a quaternary structure. Not all proteins are polymeric. Many proteins consist of a single polypeptide chain called monomeric proteins, e.g. myoglobin. • The arrangement of these polymeric polypeptide subunits in three-dimensional complexes is called the quaternary structure of the protein.

Examples

– Lactate dehydrogenase

– Pyruvate dehydrogenase

– Hemoglobin

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Functions of Proteins

Enzymatic protein

• • It accelerates the metabolic process in our cells.
  • It also accelerates the metabolic process in stomach digestion, liver functions, and blood clotting.

Hormonal protein

• • Hormonal proteins are protein-based chemicals secreted by endocrine glands.
  • By using hormonal protein, each hormone affects particular cells in the body.

Structural protein

• • Structural proteins are very important for the body because they are fibrous.
  • It helps in developing muscles, bones, skin, and cartilage.

Defensive protein

• • These defensive proteins help in developing antibodies for attacking.
  • These antibodies are developed in white blood cells to attack bacteria.

Storage protein

• • Storage protein stores minerals like potassium.
  • Storage protein contains ovalbumin and casein in milk, and egg whites.

Transport protein

• • A transport protein called calbindin is useful for calcium absorption from intestinal walls.
  • Transport proteins carry important materials to the cells of the body.

Receptor protein

• • It controls the substances that enter and leave the cells.

Contractile protein

• • It helps regulate the strength, heart speed, and muscle contractions.
  • Contractile proteins cause heart complications if the heart produces severe contractions.