How does the total enzyme concentration affect turnover number and Vmax? Enzymes with a kcat/Km ratio of about 10^8 M-1s-1 are considered to show optimal catalytic efficiency. Fumarase, which catalyzes the reversible-dehydration reaction fumarate + H2O leftrightarrow malate has a ratio of turnover number to the Michaelis-Menten constant, (kcat/Km) of 1.6 times 10^8 for the substrate fumarate and 3.6 times 10^7 for the substrate malate. Because the turnover number for both substrates is nearly identical, what factors might be involved that explain the different ratio for the two substrates? The benefit of measuring the initial rate of a reaction V_0 is that at the beginning of a reaction: A) [ES] can be measured accurately. B) changes in [S] are negligible, so [S] can be treated as a constant. C) changes in K_m are negligible, so K_m can be treated as a constant. D)_ V_0 = V_max. E) varying [S] has no effect on V_0.