Modular Enzymes
A modular enzyme is a multicomponent system or an assembly line having several component enzyme domains (also called modules). The order of enzymatic domains or modules in these multicomponent systems specifies which monomer substrates are activated, condensed and elongated. Therefore, the individual domains or components can be interchanged or replaced with functionally distinct analogues from related systems, inorder to obtain products with altered desired structure. The polyketide synthase (PKS) and non-ribosomal peptide synthetase (NRPS) are two such multicomponent systems, which can be modified by gene shuffling to provide designed diversity in complex natural products (e.g. antibiotics like penicillins, vancomycin and erythromycin).

Following three general classes of modular biocatalysts can be identified: (i) enzymes in which catalysis and substrate specifities are separable, (ii) multi substrate enzymes, in which binding sites for individual substrates are modular, (iii) enzymes, where an intermediate is transferred from one active site to another in a metabolic sequence without equilibrating with the bulk fluid. For more details on modular enzymes, the readers are advised to consult an advanced book on enzymes.