Summary

1. Most protein functions arise from their ability to specifically bind other molecules in a reversible yet specific fashion. Shape changes, triggered by the binding of other molecules, mediate protein movements and function. Allostery is a special case where a shape change induced by a ligand at one site changes the affinity at another site.

2. Enzymes are highly specific biological catalysts. The turnover number or catalytic rate constant, kcat, is the maximum number of substrate molecules that can be converted to product per molecule of enzyme per unit time.

3. The initial velocity (i.e., the rate at the start when product is absent) of many enzymes shows a hyperbolic dependence on substrate concentration. At high substrate concentrations the initial velocity approaches a limiting value Vm, as the enzyme is saturated with substrate. The substrate concentration that gives an initial velocity equal to half Vm, is the Michaelis constant KM. This indicates the enzyme's affinity for the substrate.

4. The initial velocity is related to the substrate concentration by the Michaelis-Menten equation:

5. The initial velocity is directly proportional to the enzyme concentration.

6. Some enzymes use cofactors to carry out reactions that require different properties from those of the side chains of the 20 amino acids found in proteins.

7. Enzyme activity within cells is modulated by a variety of methods that include phos-phorylation and allosteric effects.

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