Glutamine synthesis and its relation to photophosphorylation in Pisum [pea] chloroplasts. Effects of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and antimycin A

Illuminated pea (Pisum sativum) chloroplasts can convert glutamate to glutamine using ATP generated by photophosphorylation to drive the glutamine-synthetase reaction. Light-dependent glutamine synthesis is sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU), but only at concentrations higher than are necessary to suppress photoreduction of ferricyanide or phosphoglycerate. Conversely, glutamine synthesis is far more sensitive to antimycin A than is photoconversion of phosphoglycerate to triosephosphate. When 3.8 mM phosphoglycerate is supplied, glutamine synthesis is stimulated in both the presence and absence of antimycin A. These data seem to be consistent with the operation of an endogenous, DCMU-sensitive, phosphorylation process -- possibly cyclic -- which can support glutamine synthesis in white light under aerobic conditions. The stimulatory effect of phosphoglycerate suggests that noncyclic phosphorylation is initiated or accelerated when this substrate is supplied. This noncyclic process evidently provides ATP over and above the amount required for phosphoglycerate photoreduction, i.e. the $\text{ATP/e}_{2}$ ratio exceeds 1.0. The additional ATP produced under these conditions is available for glutamine synthesis and lessens its dependence on cyclically (or pseudocyclically) generated ATP.