All chemical reactions necessary for life are sufficiently slow that one or more unique enzyme catalysts are required to accelerate the reaction and make the needed product almost immediately available. Almost all enzymes are proteins that fold into domains. The majority of enzymes contains one domain (simple enzymes), while many are composed of two or more domains (allosteric enzymes and multifunctional proteins). Most enzymes are designed to function at a constant rate, but allosteric enzymes are sensitive to physiological controls, and thereby adjust their rate and determine the flux through the metabolic pathway that they control. There are two major groups of allosteric enzymes. One group is regulated by changing their affinity for one substrate, while keeping their maximum rate fairly constant (K-type enzymes). The second group also demonstrates significant changes in affinity, and in addition has large changes in the maximum rate (V-type enzymes). For cells to survive, natural selection has provided that each enzyme is always fast enough, with the slowest enzymes having a rate of ≥1 s–1
KeywordsCarbonic Anhydrase Catalytic Rate Fatty Acid Synthetase Purine Nucleoside Phosphorylase Duplicate Domain
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