Homologous Modulation
F. Xavier Malcata · 2023
Homologous modulation of activity of an enzyme requires it to be allosteric, or possess quaternary structure - and thus more than one active site per molecule; it also requires binding of substrate at a given active site to affect affinity of other active site(s) toward substrate, and/or to affect rate of conversion to product of substrate already bound at the latter active site. Two distinct (major) theories rationalize homologous modulation - sequential versus concerted transition; they lead to essentially equivalent predictions, although based on distinct underlying hypotheses. A sigmoidal rate expression is not, however, an exclusive deed of allosteric enzymes; it can also be observed in multisubstrate enzymatic reactions. The simplest example of homologous modulation involving an allosteric (or multisited) enzyme encompasses a reaction catalyzed by a dimeric enzyme, containing two identical and independent active sites per molecule - both of which are able to convert substrate to product.