Enzymes with a kcat / km ratio of about 108 m-1s-1 are considered to show optimal catalytic efficiency. fumarase, which catalyzes the reversible-dehydration reaction fumarate + h2o malate has a ratio of turnover number to the michaelis-menten constant, (kcat / km) of 1.6 × 108 for the substrate fumarate and 3.6 × 107 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?