Properties and substrate specificity of some reactions catalysed by a short-chain fatty acyl-coenzyme A synthetase from seeds of Pinus radiata

[摘要]

1. Crude extracts of seeds of Pinus radiata catalysed acetate-, propionate-, n-butyrate- and n-valerate-dependent PP_i–ATP exchange in the presence of MgCl₂, which was apparently due to a single enzyme. Propionate was the preferred substrate. Crude extracts did not catalyse medium-chain or long-chain fatty acid-dependent exchange. 2. Ungerminated dry seeds contained short-chain fatty acyl-CoA synthetase activity. The activity per seed was approximately constant for 11 days after imbibition and then declined. The enzyme was located only in the female gametophyte tissue. 3. The synthetase was purified 70-fold. 4. Some properties of the enzyme were studied by [³²P]PP_i–ATP exchange. K_m values for acetate, propionate, n-butyrate and n-valerate were 4.7, 0.21, 0.33 and 2.1mm respectively. Competition experiments between acetate and propionate demonstrated that only one enzyme was involved and confirmed that the affinity of the enzyme for propionate was greater than that for acetate. CoA inhibited fatty acid-dependent PP_i–ATP exchange. The enzyme catalysed fatty acid-dependent [³²P]PP_i–dATP exchange. 5. The enzyme also catalysed the fatty acyl-AMP-dependent synthesis of [³²P]ATP from [³²P]PP_i. Apparent K_m (acetyl-AMP) and apparent K_m (propionyl-AMP) were 57μm and 7.5μm respectively. The reaction was inhibited by AMP and CoA. 6. Purified enzyme catalysed the synthesis of acetyl-CoA and propionyl-CoA. Apparent K_m (acetate) and apparent K_m (propionate) were 16mm and 7.5mm respectively. The rate of formation of acetyl-CoA was enhanced by pyrophosphatase. 7. It was concluded that fatty acyl adenylates are intermediates in the formation of the corresponding fatty acyl-CoA.