Enzymes with a K_cat /K_m Ratio of About 10⁸ M^-1s^-1

Enzymes with a k_cat /K_m ratio of about 10⁸ M^-1s^-1 are considered to show optimal catalytic efficiency. Fumarase, which catalyzes the reversible-dehydration reaction has a ratio of turnover number to the Michaelis-Menten constant, (k_cat/K_m) 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?