Reaction of yeast fatty acid synthetase with iodoacetamide, 1. Kinetics of inactivation and extent of carboxamidomethylation

Yeast fatty acid synthetase is inactivated by several SH reagents. For the irreversible inhibition by iodoacetamide, the apparent second‐order rate constant k = 1.3 ± 0.2 M −1 s −1 was found (0 °C, pH 6.5). This value is about 125‐times higher than the rate constant for the reaction between iodoacetamide and free cysteine under the same conditions and was found to be pH‐independent in the range between pH 5 and pH 9. A possible explanation for this result might be that the SH‐group alkylated by iodoacetamide is hydrogen‐bonded to a neighbouring basic group in the protein. Of all partial activities of the synthetase, only the condensation reaction is impaired. The enzyme can be protected against iodoacetamide by prior treatment with acetyl‐CoA but not malonyl‐CoA. This indicates that iodoacetamide reacts with peripheral SH‐groups of the multienzyme complex. N ‐Ethylmaleimide, in contrast, inhibits the synthetase in a strongly pH‐dependent manner. These kinetic data support the conclusion drawn from earlier studies on the malonyl binding sites of the enzyme [Schweizer, E., Piccinini, F., Duba, C., Günther, S., Ritter, E. and Lynen, F. (1970) Eur. J. Biochem. 15 , 483–499] that the reaction of N ‐ethylmaleimide with another SH‐group of the enzyme is responsible for the inactivation. During the reaction with iodoacetamide, fatty acid synthetase activity is completely destroyed after incorporation of about three carbamoylmethyl residues per molecule of enzyme complex. At 0 °C, further alkylation by iodoacetamide could not be detected. Since yeast fatty acid synthetase is proposed to be an A 6 B 6 hexamer, the possibility of half‐site reactivity is considered.