| Abstract
| - A linked-function analysis of the allosteric responsiveness of carbamoyl phosphate synthetase(CPS) from E. coli was performed by following the ATP synthesis reaction at low carbamoyl phosphateconcentration. All three allosteric ligands, ornithine, UMP, and IMP, act by modifying the affinity ofCPS for the substrate MgADP. Individually ornithine strongly promotes, and UMP strongly antagonizes,the binding of MgADP. IMP causes only a slight inhibition at 25 °C. When both ornithine and UMPwere varied, models which presume a mutually exclusive binding relationship between these ligands donot fit the data as well as does one which allows both ligands (and substrate) to bind simultaneously. Thesame result was obtained with ornithine and IMP. By contrast, the actions of UMP and IMP togethermust be explained with a competitive model, consistent with previous reports that UMP and IMP bind tothe same site. When ornithine is bound to the enzyme, its activation dominates the effects when eitherUMP or IMP is also bound. The relationship of this observation to the structure of CPS is discussed.
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