Insights Into the Catalysis of Phosphoryl Group Transfer by Enzymes

Enzymes that catalyze phosphoryl group transfer draw from a wide variety of catalytic strategies, including transition state stabilization, acid/base chemistry, juxtaposition, and catalysis by desolvation. In this work, we investigate the rates and thermodynamics of activation of catalyzed and spontaneous phosphoryl group transfer reactions in order to understand the relative importance of those catalytic strategies. Reactions studied include the spontaneous transfer of a phosphoryl group from ATP to methanol in the presence and absence of Mg2+, the corresponding reactions catalyzed by Nacetyl galactosamine kinase, homoserine kinase, and yeast hexokinase, and the hydrolysis of phosphate monoesters and diesters in cyclohexane and other organic solvents. We show that N-acetyl galactosamine kinase, homoserine kinase, and yeast hexokinase accelerate phosphoryl transfer from ATP to an alcohol acceptor group by factors of 1012-1014, relative to the spontaneous transfer of a phosphoryl group from Mg-ATP to methanol, which occurs at a rate of 3.9 x 10-9 s-1M-1. These kinases achieve their rate enhancements by decreasing ΔH‡ to an extent comparable to other enzymes (8-13 kcal./mol), but they also increase TΔS‡ by 6-9 kcal./mol, in accord with the possibility that juxtaposition is a general requirement for bi-substrate enzymes. Consistent with mechanistic suggestions, the relative contributions of ΔH‡ and TΔS‡ to the rate enhancement differ for the three kinases. N-acetyl galactosamine kinase, which lacks a catalytic base, increases TΔS‡ to the greatest extent, whereas homoserine kinase and hexokinase decrease ΔH‡ and seem to rely more heavily on acid/base catalysis or transition state stabilization. We also show that extraction of neopentyl phosphate monoesters and diesters into cyclohexane accelerates their hydrolyses by factors of 2 x 1012 and 4 x 109, respectively. The hydrolysis of the monoester is accelerated to a similar extent in polar solvents like DMSO. For phosphate monoester hydrolysis in cyclohexane, the effect of desolvation was to increase TΔS‡, whereas for phosphate diester hydrolysis in cyclohexane and phosphate monoester hydrolysis in DMSO, the effect of desolvation was to decrease ΔH‡. These rate enhancements approach those generated by many enzymes, and imply that desolvation might be a powerful means of catalysis for phosphate monoesterases and phosphate diesterases.Doctor of Philosoph