Effect of the side chain structure of coenzyme Q on the steady state kinetics of bovine heart NADH: coenzyme Q oxidoreductase

Steady state kinetics of bovine heart NADH: coenzyme Q oxidoreductase using coenzyme Q with two isoprenoid unit (Q₂) or with a decyl group (DQ) show an ordered sequential mechanism in which the order of substrate binding and product release is NADH-Q₂ (DQ) -Q₂H₂ (DQH₂)-NAD⁺ in contrast to the order determined using Q₁ (Q₁-NADH-NAD⁺-Q₁H₂) (Nakashima et al., J. Bioenerg. Biomembr.34, 11–19, 2002). The effect of the side chain structure of coenzyme Q suggests that NADH binding to the enzyme results in a conformational change, in the coenzyme Q binding site, which enables the site to accept coenzyme Q with a side chain significantly larger than one isoprenoid unit. The side chains of Q₂ and DQ bound to the enzyme induce a conformational change in the binding site to stabilize the substrate binding, while the side chain of Q₁ (one isoprenoid unit) is too short to induce the conformational change.