Abstract
Acetyl coenzyme A carboxylase, a biotin-containing enzyme, plays a critical role in the control of fatty acid biosynthesis. Evidence has been presented which indicates that the regulation at this carboxylation step is achieved both by changes in the catalytic efficiency of carboxylase molecules and by changes in the quantity of the enzyme. (1) Kinetic and other studies with the purified liver carboxylase have shown that the main role of the allosteric activator, citrate, in the carboxylase catalysis is to keep the carboxybiotinyl enzyme in active conformation by shifting the equilibrium between the active and inactive species of the carboxylated enzyme. (2) The mechanism by which the carboxylase content of the liver is regulated in various metabolic states has beeh investigated by combined immunochemical and isotopic techniques. The results of these studies indicate that under steady-state conditions the carboxylase content is controlled mainly by changes in the rate of enzyme synthesis, while under nonsteady-state conditions both the rate of enzyme synthesis and the rate constant of enzyme degradation are altered to change the carboxylase content. (Received July 18,1972)
