This document discusses protein engineering, which uses recombinant DNA technology to modify protein structure and function. It describes several methods of protein engineering, including site-directed mutagenesis, error-prone PCR, and DNA shuffling. The objectives of protein engineering are to improve protein stability, modify cofactor requirements, increase enzyme activity, and modify enzyme specificity. As an example, the document discusses how site-directed mutagenesis was used to increase the stability of streptokinase by replacing lysine residues susceptible to cleavage by plasmin.