Question

Provide specific example for allosteric enzymes. Do allosteric enzymes follow Michaelis-Menten? Explain

Answer 1

The word "allosteric" is derived from two Greek words: allos , meaning other, and stereos , meaning site.

Allosteric enzymes have the ability to respond to several different conditions in their environments. Every enzyme contains an active site, the location on the enzyme where it catalyzes its specific reaction. Allosteric enzymes contain a second type of site, called an allosteric site. The allosteric site, through its binding of a nonsubstrate molecule, influences (enhances or impairs) the activity of the enzyme.

The example of an allosteric enzyme is aspartate trascarbamoylase. The enzyme catalyzes the first step in the synthesis of pyrimidines. The enzyme functions to catalyze the condensation of aspartate and carbamoyl phosphate to form Ncarbamoylaspartate and orthophosphate. The enzyme ultimately catalyzes the reaction that will yield cytidine triphosphate (CTP). This allosteric enzyme is unique in that for high products of the final product CTP, the enzyme activity is low. However, for low concentrations of the final product CTP, the enzymatic activity is high. The allosteric nature is thus represented as the CTP molecule has a odd configuration or shape that is unlike the substrates. Rather than binding to the active site, CTP binds to the allosteric site. Thus, CTP functions as an allosteric inhibitor decreasing the enzymatic activity of the enzyme. This enzyme also has separate regulatory and catalytic subunits on separate polypeptide chains. There are instances though when CTP concentrations remain high and cells in the body need more enzyme. This is when a different allosteric molecule ATP functions to attach to the allosteric site and functions as enzyme activator enhancing the activity of the enzyme. Thus, even with high concentrations of CTP, the enzyme activity could be enhanced because of ATP, which also acts on the allosteric site.

Allosteric enzymes do not follow the Michaelis-Menten Kinetics. This is because allosteric enzymes have multiple active sites. These multiple active sites exhibit the property of cooperativity, where the binding of one active site affects the affinity of other active sites on the enzyme. it is these other affected active sites that result in a sigmoidal curve for allosteric enzymes.