The α-Chymotrypsin-catalyzed Hydrolysis of a Series of Acylated Glycine Methyl Esters. II. Behavior at Low and High Substrate Concentrations*

Purification and properties of threonine aldolase from Clostridium pasteurianum

1. Threonine aldolase was purified about 200-fold in 10% yield from Clostridium pasteurianum and its properties were examined. The final preparation gave three bands after ionophoresis on polyacrylamide gel. 2. The purified enzyme was shown to produce glycine and acetaldehyde in stoicheiometric amounts from threonine. The reverse reaction was demonstrated qualitatively. 3. The enzyme has a broad pH optimum at 6.5-7.0. 4. The enzyme is highly specific for l-threonine. 5. The enzyme is completely inhibited by 1mm concentrations of hydroxylamine and semicarbazide. Activity is decreased to 20% of the original by treatment with cysteine plus mercaptoethanol; most of the loss is regained on incubation with pyridoxal phosphate. It is concluded that pyridoxal phosphate is a prosthetic group. 6. The relationship between velocity and substrate concentration is atypical but indicates a K(m) value of 0.42mm. 7. The enzyme was demonstrated in several other strictly anaerobic bacteria.

The alpha-chymotryptic ydrolysis of glycine esters

1. The alpha-chymotrypsin-catalysed hydrolysis of N-acetylglycine ethyl and thiolethyl esters was investigated at pH7.90 and 25 degrees over a wide range of substrate concentrations. 2. The Lineweaver-Burk plots for these substrates are markedly curved, and it is shown that the curvature is due solely to the ;enzyme-blank' reaction. The rate of this reaction is proportional to free enzyme concentration in the range 10-100mum, with a pseudo-first-order rate constant of approx. 1x10(-3)sec.(-1). Correction for this reaction by the procedure described leads to linear plots. It is shown that the significance of the enzyme-blank reaction depends on the value of k(0)/K(m) for the substrate under investigation. 3. Interpretation of the curvature in the Lineweaver-Burk plots by previous workers in terms of activation by excess of substrate is shown to be erroneous. 4. Values of K(m) 387mm and k(0) 0.039sec.(-1), and K(m) 41mm and k(0) 0.23sec.(-1), were obtained for the ethyl and thiolethyl esters of N-acetylglycine respectively. The literature values for the methyl esters of N-acetyl- and N-propionyl-glycine have been corrected by the procedure described. The new values agree much better with current theories of alpha-chymotrypsin mechanism and specificity. 5. The kinetic parameters for the ethyl and thiolethyl esters indicate the absence of an electrophilic component in the catalytic mechanism of alpha-chymotrypsin, and the importance of the ester function in substrate binding.