- As biological catalysts – enzymes.
- Provide structural framework of cells.
- Act as transport media in bloodstream.
- Act as hormones.
- Perform mechanical work in skelatal muscle and heart.
- Act as antibodies.
- Regulate gene expression.
- Short sequences of amino acids linked covalently, are known as peptides.
- Peptide bond is formed by reaction of amino group of one amino acid with the carboxyl group of other.
- On the basis of number of amino acids they are termed di, tri, tetra- peptides and so on.
Biologically active peptides
Bonds Responsible for Protein Structure.
- Covalent bond
- Peptide bonds
- Disulphide bond
2. Noncovalent bond
- Hydrogen bond
- Hydrophobic bond or interaction
- Electrostatic or ionic bond
- Van der Waals interactions.
- Peptide bonds (–CO-NH–) : bond formed by the condensation of the amino group of one amino acid with the carboxyl group of another amino acid with a removal of a water molecule
- Disulfide bond (-S-S-) : bond formed between the sulfhydryl group (-SH) of side chain of cysteine residues.
- Hydrogen bond : formed between -NH and -CO groups of peptide bond by sharing single hydrogen. Side chains of some amino acids can also form hydrogen bond.
- Hydrophobic bond or interaction : formed by interaction between nonpolar hydrophobic R groups (side chain) of amino acids like alanine, valine, leucine, isoleucine, methionine, phenylalanine and tryptophan.
- Electrostatic or ionic bond or salt bond : formed between oppositely charged groups such as amino (NH3+) terminal and carboxyl (COO–) terminal groups of the peptide and oppositely charged R-groups of polar amino acid residues.
- Van der Waals interactions : include both an attractive and a repulsive forces (between both polar and nonpolar side chain of amino acid residues).
Structure – function relationship
- The following three proteins are considered; each belongs to a different class in the functional classification.
- Transport proteins
- Structural proteins
- The first step in enzymatic catalysis is the binding of the enzyme to the substrate.
- This, in turn, depends on the structural conformation of the active site of the enzyme, which is precisely oriented for substrate binding.
- Eg. Carbonic anhydrase catalyses the reversible hydration of carbon dioxide.
- Eg. Haemoglobin – sickle cell anemia (HbS) This is formed when the glutamate residue at position 6 in the β chain is replaced by valine.
- This amino acid residue is present on the surface of the haemoglobin molecule.
- Replacement of the polar glutamate by nonpolar valine alters the surface properties of the haemoglobin molecule.
- The nonpolar valine residue on the surface of one molecule attracts the nonpolar residue of another haemoglobin molecule.
- This starts a chain reaction resulting in the aggregation of several hemoglobin molecules, which form a fibrous structure that distorts the erythrocyte into a sickle-shaped cell
- Therefore, aggregation of haemoglobin molecules and sickling of erythrocytes occur when haemoglobin is present in the deoxygenated form i.e. at low oxygen tension.
- Sickled erythrocytes are susceptible to premature destruction.
- Rapid destruction of erythrocytes causes haemolytic anaemia
- Collagen is the most abundant protein in mammals and is the main fibrous component of skin, bone, tendon, cartilage and teeth.
- In vitamin C deficiency, failure of hydroxylation of proline/lysine leads to reduced hydrogen bonding and consequent weakness of collagen.
- The quarter staggered triple helical structure of collagen is responsible for its tensile strength.
Properties of proteins
- Colloidal nature
Colloidal protein molecules exert osmotic pressure.
Colloidal osmotic pressure or oncotic pressure
exerted by protein maintain blood volume.
- Molecular weight
Albumin = 6,9000
γ-Globulin = 1,60,000.
globular proteins, such as, albumin have higher solubility than elongated fibrous proteins. Moreover, smaller molecules are more soluble than larger molecules.
- Shape of the protein
Scleroproteins like keratin, collagen are in the form of fibers. While soluble proteins tend to be of rounded shape and are called globular proteins.
- Amphoteric nature
- Isoelectric pH of the protein
- Hydration of proteins
Shell-like layer of water, called the “solvation layer” or water envelope isheld around each protein particle in an aqueous medium.
Denaturation of protein
Disorganization of primary, secondary, tertiary and quaternary structure by breaking of Hydrogen bond, ionic bond and hydrophobic bond without breakage of any peptide linkage.
Significance of denaturation
Digestibility of native protein is increased.
– in blood analysis to eliminate the proteins of
For example boiled egg.
Other Biochemistry Notes
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