Multienzyme System

Some enzyme function more effectively when their subunits, of which they are built, remain in physical association with each other. Such enzyme complexes respond to change in substrate concentration showing varying catalytic efficiency. Three levels of organization for multienzyme  system have been recognized. The first level of organization represents a condition when the enzymes of their subunits occur as independent molecular entities are not physically associated with each other. In a biosynthetic sequence, when a substrate is attacked by an enzyme, its product becomes the substrate for the next enzyme, and thus a multienzyme system carries out a sequence of enzyme catalyzed steps . This is a case of feedback regulation. However, the end product, if found in excess, binds the first enzyme E1 causing feeb-back inhibition or retroinhibition thus the end product acts as a negative allosteric effector, but there may be small metabolites that may act as positive allosteric effectors which may have an affinity for the catalytic site or the allosteric site. A second level of organization is seen in case of fatty acid synthetase  complex in the cytosol of liver cells and other types of cells that carry out synthesis of fatty acids. The fatty acid synthetase composed of seven enzyme units that remain clustered together and if any subunit is dissociated, the catalytic function of the whole complex is jeopardized. A third type of organization is seen in the membranes of some organelles like chloroplasts and mitochondria, where the enzymes are associated with the inner membrane that carries out the function of electron transport in a sequential manner. The multienzyme system are quite complicated in their behavior since each enzyme may have its own km value, hence they do not follow Michaelis- Menten kinetics.