CHAPTER 1 - THE HIERARCHY OF ENZYME STRUCTURES

Abstract

Enzymes share the property with other globular proteins of being are linear polymers of amino acids folded in an irregular but highly specific manner to form fairly compact three-dimensional structures. One polypeptide chain usually gives rise to one globular unit. The simplest enzymes such as lysozyme, trypsin, or ribonuclease consist of a single polypeptide chain or at least are derived from one chain. The latter case is exemplified by chymotrypsin, which is synthesized as single-chain chymotrypsinogen and then converted to α-chymotrypsin through several proteolytic fissions. The α-chymotrypsin molecule consists of three unlike polypeptides in one globular entity. The structural organization within the above monomeric enzymes and other globular proteins has distinct levels. According to the classical categorization, the primary structure is the sequence of amino-acid residues in the polypeptide chain. The secondary structure comprises various ordered arrangements of the polypeptide backbone such as α-helices, reverse turns, and β-pleated sheets. The term tertiary structure denotes the overall three-dimensional conformation of the polypeptide chain. More recently, as a result of the accumulation of data obtained by X-ray crystallography, two further levels of organization emerged between the secondary and tertiary ones. Supersecondary structures are aggregates of secondary structures apparently preferred in several proteins for reasons of thermodynamic or kinetic stability.