What does the initial enzyme rate of reaction (vo) depend upon at low substrate concentrations?

At low substrate concentrations, the initial enzyme rate of reaction (vo) predominantly follows first-order reaction kinetics. This means that the rate of reaction is directly proportional to the concentration of the substrate. Essentially, as the substrate concentration increases, more substrate molecules are available to collide with enzyme molecules, leading to a higher rate of reaction.

In this context, at low substrate concentrations, the enzyme active sites are not yet saturated, allowing the reaction rate to increase more linearly with increases in substrate concentration. Therefore, the initial rate (vo) is highly dependent on the availability of substrate; the rate will increase as substrate concentration increases until a point is reached at which saturation occurs.

In contrast, zero-order kinetics would indicate that the reaction rate does not change with changes in substrate concentration, which is not the case at low substrate levels. Second-order kinetics implies that the rate depends on the concentration of two reactants, which does not apply when only the substrate is being considered. While enzyme concentration can influence the maximum rate of a reaction, the dependence on substrate concentration specifically at low levels aligns with first-order kinetics.