The module sets out to explain the molecular basis of the often complex catalytic mechanisms of enzymes in biological systems. Covered are the underlying principles of enzyme catalysis and techniques for the study of enzyme mechanism and structure. These provide a foundation for discussions of the catalytic and cellular mechanisms of proteinase families such as the serine and metalloproteinases as well as mechanism-based drug design for the retroviral aspartic proteinase, HIV-1pr. Covered also are molecular motors , complex nanomachines involved in vesicle transport, ATP synthesis and DNA replication. Finally, the biosynthesis of the signalling molecule nitric oxide and the P450s involved in the metabolism of drugs and other xenobiotics are presented. An extended practical based on the kinetics of a model enzyme, chymotrypsin, helps underpin concepts learnt in the module.