Serine transhydroxymethylase. Relaxation and transient kinetic study of the formation and interconversion of the...

Serine transhydroxymethylase forms three enzyme-glycine complexes which absorb at 3.43, 425, and 495 nm. Temperature-jump studies show three relaxations. Two of the relaxations are observed at both 343 and 425 nm but not at 495 nm. A slower third relaxation is observed only at 495 nm. The absorbance changes for the two relaxations observable at 343 and 425 nm are in opposite directions suggesting that these relaxations are both attributable to the inter conversion of enzyme speiies absorbing at these two wavelengths. The following mechanism is proposed to explain the relaxation effects. See journal for formula. The forward and reverse rate constants for the first step are 7 X 10-4 M-MINUS1 sminus-1 and 1200 Sminus-1, respectively. The forward and reverse rate constants for the second step are 3200 and 1300 sminus-1, respective.y Stopped flow studies on the rate of formation and breakdown of the complexes absorbing at 343 nm (EX) and 425 nm (EY) are in agreement with the proposed mechanism. Stopped flow studies gave a Kobs of 0.1 Sminus-1 for the formation of the 495-nm absorbing complex. This is compared to the reciprocal relaxation time of 200 Sminus-1 observed in the temperature-jump studies. The addition of tetrahydrofolate to this system increased the Kobs to 275 Sminus-1 in the stopped flow studies and the reciprocal relaxation time to 800 Sminus-1 in the temperature-jump studies. The data do not permit a simple interpretation of the relationship of the 495-nm absorbing complex to those adsorbing at 343 and 425 nm. However, the data do support the interpretation that the ability of tetrahydrofolate to increase by 3 orders of magnitude the enzyme-catalyzed rate of exchange of the alpha-hydrogen of glycine with protons of the solvent is attributable to an increased rate of formation of the 495-nm complex.