PLANT PHYSIOLOGY , Vol 109, Issue 4 1441-1451, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Subsaturating Ribulose-1,5-Bisphosphate Concentration Promotes Inactivation of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase (Rubisco) (Studies Using Continuous Substrate Addition in the Presence and Absence of Rubisco Activase)
A. R. Portis Jr, R. M. Lilley and T. J. Andrews
Research School of Biological Sciences, Australian National University, Canberra, Australia (A.R.P., T.J.A.)
We developed a continuous-addition method for maintaining subsaturating
concentrations of ribulose-1,5-bisphosphate (RuBP) for several minutes,
while simultaneously monitoring its consumption by
ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). This method
enabled us to observe the effects of subsaturating RuBP and CO2
concentrations on the activity of Rubisco during much longer periods than
previously studied. At saturating CO2, the activity of the enzyme declined
faster when RuBP was maintained at concentrations near its Km value than
when RuBP was saturating. At saturating RuBP, activity declined faster at
limiting than at saturating CO2, in accordance with previous observations.
The most rapid decline in activity occurred when both CO2 and RuBP
concentrations were subsaturating. The activity loss was accompanied by
decarbamylation of the enzyme, even though the enzyme was maintained at the
same CO2 concentration before and after exposure to RuBP. Rubisco activase
ameliorated the decline in activity at subsaturating CO2 and RuBP
concentrations. The results are consistent with a proposed mechanism for
regulating the carbamylation of Rubisco, which postulates that Rubisco
activase counteracts Rubisco's unfavorable carbamylation equilibrium in the
presence of RuBP by accelerating, in an ATP-dependent manner, the release
of RuBP from its complex with uncarbamylated sites.
