Since genes encode enzymes, the changes in gene certainly bring about alteration in enzyme structure. In addition to methods available for gene manipulation alteration of genes by site directed mutagenesis (see Immobilized cell culture) for enzyme engineering has become much popular. Thus, site-directed mutagenesis produces single amino acid substitutions in the primary structure of enzymes.
In recent years, the term "enzyme engineering" is used to denote the modification of enzyme structure by alteration of genes which code enzymes. An enzyme produced by a modified gene is structurally new which has great promise to create a new enzyme. Enzyme engineering embraces the (i) production of new enzymes, (ii) study of structural feature related to stability (iii) increase in stability by changing amino acid composition, and (iv) production of stable enzymes by genetically engineered microbial cells.