Most enzymes are proteins but some are RNA.
An enzyme contains an active site, which comprises two functional parts: a substrate-binding region and a catalytic region (Fig. 3.23).
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The amino acids that make up the active site do not need to be adjacent in the polypeptide chain, but in fact can be from different regions of the chain that are brought together by protein folding (Fig. 3.24)
A cellular enzyme must be able to catalyze its reaction in an environment normally encountered in a cell, i.e., in aqueous solution, at pH 6.5 to 7.5, at 37C.
Some enzymes change conformation in response to substrate binding (induced fit model) (Fig. 3.25) but after the product is released, the enzyme is the same as before binding the substrate.
The rate at which an enzyme operates is dependent on the binding, catalysis and release constants:
binding catalysis
release
E + S <->
ES <-> EP
<-> E + P
If all enzyme sites are saturated with substrate, then an enzyme will function at its maximum velocity (Vmax, Fig. 3.26b).
The substrate concentration at which the enzyme is running at half-maximal velocity is called the Km and is a combination of the rate limiting binding, catalysis or release constants (Fig. 3.26b).
The velocity or rate of reaction at any given substrate concentration can be determined by the equation:
velocity = Vmax * [S] / ( [S] + Km ), where [S] is the substrate concentration.Regulation of enzyme activity
Substrate concentration (Fig. 3.26b).
Allosteric release of catalytic subunits (Fig. 3.27a).
Allosteric transition between active and inactive states (Fig. 3.28).
Cooperative binding of ligands (Fig. 3.29).
Protein phosphorylation and dephosphorylation (Fig. 3.30).
Proteolytic activation (Fig. 3.31).
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