Regulation of stromal sedoheptulose 1,7-bisphosphatase activity by pH and Mg2+ concentration.
1984; Elsevier BV; Volume: 259; Issue: 6 Linguagem: Inglês
10.1016/s0021-9258(17)43164-4
ISSN1083-351X
AutoresIan E. Woodrow, Denis J. Murphy, Erwin Latzko,
Tópico(s)Light effects on plants
ResumoA scheme is proposed for the regulation of stromal sedoheptulose 1,7-bisphosphatase activity which enlarges upon a previously elaborated mechanism (Woodrow, I. E., and Walker, D. A. (1983) Biochim.Biophys.Acta 722, 508-516).The latter involves oxidized (inactive) and reduced (active) enzyme forms.Both the free enzymes and the enzyme-substrate complexes undergo slow oxidation/reduction.This study examines the behavior of the system under pH and M 8 + concentration regimes that are likely to occur in the chloroplast stroma.The control of enzyme activity by pH can be described in terms of each free enzyme and enzyme-substrate complex existing in protonated and nonprotonated forms.The molecular dissociation constants for each protonation reaction were calculated from kinetic data.Mg"+ concentration changes modulate these constants.Under conditions that are likely to obtain in the stroma in the dark, the model predicts that approximately 99.1% of the enzyme will be in the inactive forms.Such inactivation is important since it would prevent the reductive pentose phosphate pathway from operating in darkness.Upon illumination the stromal pH increases by about one pH unit from a near neutral state in darkness (Heldt et al., 1973).The mechanism is believed to involve the "pumping" of protons from the stroma into both the intrathylakoid space (Neumann and Jagendorf, 1964;Trebst, 1974) and the cytosol (Heber and Krause, 1971;Gimmler et al., 1975).The process of altering the pH by using weak acids was employed to investigate the role of stromal pH changes in controlling the flux through the reductive pentose phosphate pathway of photosynthesis during illumination.Where the dissociated and nondissociated forms of weak acids could cross the chloroplast envelope, they constituted a proton shuttle (Purczeld et al., 1978;Heber et al., 1979) and effectively equilibrated the stroma with the surrounding medium (Heber et al., 1979).Addition of weak acids to a medium containing isolated chlo-
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