Question

Glycine residues occur in a higher percentage in proteins than any other amino acid.  Suggest two reasons why.

Answer 1

Glycine is the smallest of the known conventional 20 amino acids. The chemical formula is :

CH3 - CH (COOH) (NH3)

We can observe that Glycine has only a single methyl group as its functional group. Due to this it is very small. This small structure is the reason for its high percentage in proteins which caters the following two functions :

A. A protein chain can be a very long one . It needs to be folded in a very compact manner so that it can fit inside several organelles such as endoplasmic reticulum, cell membrane, mitochondria, nucleus, etc. The compact form of glycine aids in the formation of the alpha-helix in secondary protein structure. Glycine is present at the specific turns of the helix and facilitates the bending of the protein into a helix turn.

B. We know that enzymes are proteins. Enzymes contain a specific site called catalytic site or active site which binds to the substrate and aids in the formation of product. The active site of the enzyme requires to undergo conformational bending or changes in order to interact with substrate, catalyse the reaction and form the product. This conformational change requires flexibility of the protein in the active site. This flexibility is provided by glycine since it can induce a turn or bend in the active site owing to its compact small structure.

Since a majority of proteins have exist in the alpha helix secondary structure form and all the enzymes contain active sites, the glycine residues occur in a higher percentage in proteins than any amino acid.

PS -Secondary structure refers to the three dimensional arrangement and folding of proteins which majorly involve the formation of Hydrogen bonds.

Enzyme active site is the region of an enzyme where substrate undergoes chemical reaction to give rise to product.