A regulatory enzyme is an enzyme in a biochemical pathway which, through its responses to the presence of certain other biomolecules, regulates the pathway's activity. This is usually done for pathways whose products may be needed in different amounts at different times, such as hormone production. Regulatory enzymes exist at high concentrations (low Vmax) so its activity can be increased or decreased with changes in substrate concentrations.
The enzyme which catalyse chemical reactions again and again are called regulatory enzymes.
Regulatory enzymes are of two types: Allosteric enzymes and covalently modulated enzymes.
The allosteric enzymes has two binding sites. One of the binding sites is for the substrate of the enzyme, the other site is for small molecules called effectors which modulates the enzymes activity.effectors are non-covalently linked to the enzyme at its allosteric site (site of enzyme where the effector binds) and its interaction with the enzyme is reversible. Based on modulation, allosteric enzymes can be grouped into two groups: 1.homotropic allosteric enzyme and 2.heterotropic allosteric enzymes. In the homotropic allosteric enzyme both the substrate and the effector plays part in the modulation of the enzyme, which in turn affects the enzyme catalytic activity. In the heterotropic form it is only the effector that performs the role of modulation. At this junction the allosteric site of the enzyme could range from one(monovalent) or from two and above (polyvalent). Usually regulatory enzyme starts the catalysis of multienzyme reaction e.g. gycolysis, fat synthesis e.t.c. An example of an allosteric enzyme is aspartate transcarbamoylase. This enzyme catalyzes the synthesis of pyrimidine nucleotides.
Covalently Modulated Enzyme: Here,active and inactive form of the enzyme are interconverted by covalent modification of their structures which are catalysed by other enzymes. A typical example is glycogen phosphorylase which occurs in two forms, Phoshorylase A, the more active one and Phosphorylase B, the inactive one.