Principles of Biochemistry (2+1)

5.2.2 Forces that stabilize the protein structure

Several non-covalent bonds stabilize the protein conformation. These forced are: hydrogen bonds, hydrophobic interactions, Van der Waals forces, electrostatic interactions, and disulphide bonds.

a) Hydrogen Bonds:

Hydrogen bonds are formed between the amide nitrogen and the carbonyl oxygen of the peptide backbone as well as groups present in the side chain.

 

b) Hydrophobic Interactions

Hydrophobic interactions result when two nonpolar side chains are close to each other. The side chains of aspartic acid, glutamic acid, tyrosine, histidine, serine and threonine are all capable of hydrogen bond formation.  In aqueous solution, many proteins have their polar R groups outward, towards the aqueous solvent (water is polar) and their nonpolar R groups inward (away from polar water molecules). The nonpolar R groups then interact with each other. Hydrophobic interactions are common between phenyl rings and alkyl side chains. Although they are weaker than hydrogen or electrostatic bond, they are a significant force in some proteins because there are so many of them; their cumulative effect can be greater in magnitude than the effects of hydrogen bonding.

c) Van der Waals Interactions:

Vander der Waals forces, which are extremely weak and act only over extremely short distances, include both an attractive and a repulsive component.The attractive force involved interaction between induced diploes formed by momentary fluctuations in the electron distribution in nearby atoms. The repulsive forces come into play when two atoms come so close that their electrons orbital overlap.

d)Electrostatic Bonds or ionic bonds:

Electrostatic bonds link surface residues. These bonds link oppositely charged R groups of amino acid residues and the charged α- carboxyl and α amino terminal residue. For example, the R group of lysine and aspartate or glutamate interact electro statically to stabilize proteins.

                                                                                                   

e) Disulfide Bonds:

Covalent disulfide bonds can be formed between cysteine residues present in the same (an intramolecular disulphide) or different polypeptides (an intermolecular disulphide bond). These disulfide bonds confer additional stability to specific conformations of proteins such as enzymes (e.g. ribonuclease) and structural proteins (e.g. Keratin).