SITE-DIRECTED MUTAGENESIS OF A SERINE RESIDUE IN CINNAMYL ALCOHOL-DEHYDROGENASE, A PLANT NADPH-DEPENDENT DEHYDROGENASE, AFFECTS THE SPECIFICITY FOR THE COENZYME

Using recombinant cinnamyl alcohol dehydrogenase isoform 3 (CAD2, EC 1 .1.1.195), an NADPH-dependent aromatic alcohol dehydrogenase involved in lignification in vascular plants, we have investigated the detailed steady-state kinetic mechanism of CAD2 and the role of a serine resid ue in determining the cofactor specificity of CAD2. Site-directed muta genesis (S212D) and overexpression of the WT and mutant S212D forms of CAD2 in Escherichia coli, followed by kinetic studies on the purified WT and mutant proteins, confirmed the involvement of S212 in recogniz ing the phosphate group of NADPH and provided information on the struc tural requirements for NADPH specificity, From substrate kinetic patte rns and product inhibition studies both WT and S212D mutant farms of C AD2 have been shown to follow rapid equilibrium random bireactant kine tics with the value of the interaction factor (a) for WT (0.25) being significantly less than that for S212D CAD2 (0.45). The changes in bin ding energy arising from the mutation on the binding of the 2'-phospha te site of the coenzyme were assessed. A marked degree of physical int eraction was detected between the enzymatic binding sites of the conif eryl alcohol substrate and the 2'-phosphate binding region, which are quite distant in the three-dimensional structure, The inhibition by 2' ,5'-ADP and 5'-AMP was found to be weak for both WT and S212D CAD2, St rong substrate inhibition was detected for CAD2, and its implications for plant physiological studies were assessed. The overall catalytic e fficiency [k(cat)/(K-m(A) x K-m(B))] for CAD2 with NADP(+) as coenzyme is decreased 2.2 x 10(3)-fold by the single mutation S212D, while the re is no significant change in this parameter with NAD(+) as coenzyme. The S212D mutation allows CAD2 (normally using NADPH) to function wit h NADH as coenzyme at 1/25 the rate of WT with NADPH, indicating that CAD2 would provide a good basis for any multiple-mutation engineered s witch in coenzyme usage from NADPH to NADH to produce a metabolic prob e of plant physiology.