Some enzymes possess additional sites known as allosteric sites (allo – other, steric -solid or shape) besides the active site. Such enzymes are known as allosteric enzymes. Allosteric enzymes are usually multiple subunit enzymes, and allosteric sites may be located on different subunits. The binding of regulatory molecules and modulators or modifiers or effectors can either enhance the activity of enzyme (allosteric activation) refer Figure 6.27 – the effector is called positive effector – or inhibit enzyme activity (allosteric inhibition) refer Figure 6.27 – the effector is called negative effector.