Proteins serve many functions in life. One class of proteins,called enzymes, are biochemical catalysts; that is, they speed up the rate of biochemical reactions without being used up in the reactions. If it were not for enzymes, metabolism could not proceed and life could not exist. Other types of proteins, called transport proteins, reside within the lipid bilayer of membranes. These proteins ferry specific substances across membranes or make channels through which the substances may pass. Since it is important that the cell's environment remain distinct from the outside environment yet obtain certain substances from outside the cell, transport proteins are vital for the life of the organism also. Proteins also provide the machinery for movement both at the organismal level (muscles ) and at the cellular level (cytoskeleton). In animals, proteins also serve to fight off infections (antibodies), to regulate development (peptide hormones such as insulin), and are structural components of feathers, hair, nails, and the dermis of the skin.

Proteins carry out most of these functions by first binding to specific molecules. Once this occurs, the protein shape is altered in such a way that the bound substance is affected . Transport proteins move the bound substance across the membrane; enzymes change the bonds of the bound substance, resulting in a product formation; antibodies bind to the infectious particle causing its destruction. Let's look at how enzymes operate.