Concept 03 / 05Biomolecules
Amino Acids and Proteins: Structure and Denaturation
α-amino acids (essential and non-essential, neutral, acidic and basic), zwitterions and the isoelectric point, the peptide bond, fibrous and globular proteins, primary, secondary (α-helix and β-pleated sheet), tertiary and quaternary structure, and denaturation.
Amino acids contain an amino group and a carboxyl group. Those obtained by hydrolysis of proteins are α-amino acids, , with about 20 different side chains R. Glycine (R = H) is the only achiral one; all others are optically active, and natural ones have the L-configuration.
Classification. Neutral (equal numbers of and COOH groups: glycine, alanine), acidic (more COOH: aspartic acid, glutamic acid) and basic (more : lysine, arginine). Essential amino acids cannot be made by the body and must come from the diet (valine, leucine, isoleucine, arginine, lysine, threonine, methionine, phenylalanine, tryptophan, histidine); non-essential ones are synthesised in the body.
Zwitterion. In aqueous solution the carboxyl group loses a proton to the amino group, giving a dipolar ion . This explains why amino acids are colourless crystalline solids with high melting points, more soluble in water than in organic solvents, and amphoteric (they react with both acids and bases). The pH at which the net charge is zero is the isoelectric point (pI); for a neutral amino acid, . At pH below pI the cation dominates and moves to the cathode; above pI the anion moves to the anode.
Peptide bond. The amide link formed between the COOH of one amino acid and the of another, with loss of water. Two amino acids give a dipeptide, three a tripeptide; more than ten a polypeptide; a polypeptide with more than about 100 residues (mass above 10000 u) is called a protein. A chain of residues contains peptide bonds.
Types. Fibrous proteins have parallel chains held by hydrogen and disulfide bonds; they are usually insoluble in water (keratin in hair and wool, myosin in muscle). Globular proteins have chains folded into spheres and are usually water-soluble (insulin, albumins).
Levels of structure.
Primary: the sequence of amino acids. A change of even one residue changes the protein (sickle-cell haemoglobin differs by one amino acid).
Secondary: the shape of the backbone. In the α-helix, a right-handed helix, each -NH is hydrogen-bonded to the C=O of the fourth residue along the chain. In the β-pleated sheet, chains lie side by side, stretched almost fully, and are joined by intermolecular hydrogen bonds.
Tertiary: the overall folding of the chain, giving fibrous or globular shape, stabilised by hydrogen bonds, disulfide links, van der Waals forces and electrostatic attractions.
Quaternary: the arrangement of two or more polypeptide subunits (haemoglobin has four).
Denaturation. Heat, a change in pH or certain chemicals disrupt the hydrogen bonds, so the helices uncoil and the protein loses its biological activity. The secondary and tertiary structures are destroyed but the primary structure is not. Examples: coagulation of egg white on boiling, curdling of milk (lactic acid from bacteria).
Key Takeaways:
- α-amino acids exist as zwitterions; pI is the average of the two values for a neutral amino acid.
- The peptide bond is an amide link; residues have peptide bonds.
- 1° sequence, 2° α-helix or β-sheet, 3° folding, 4° subunits.
- Denaturation destroys 2° and 3° structure, never the 1° sequence.
- Pitfall: glycine is the only common amino acid that is not optically active.
Brain-Teaser 1: For glycine, and . Find the isoelectric point, and state the direction of migration at pH 3 in electrophoresis.
Solution: . At pH 3, which is below pI, glycine is mainly the cation (net positive), so it moves towards the cathode.
Brain-Teaser 2: How many different tripeptides can be formed from glycine, alanine and valine if (a) each is used exactly once, (b) repetition is allowed?
Solution: A peptide has direction (N-terminus to C-terminus), so order matters. (a) . (b) .
Brain-Teaser 3: A hexapeptide of molar mass is completely hydrolysed. What is the total mass of the amino acids obtained from one mole?
Solution: A hexapeptide has 5 peptide bonds; hydrolysis adds 5 mol of water. The total mass of amino acids is .
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