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• W2110777712 endingPage "190" @default.
• W2110777712 startingPage "151" @default.
• W2110777712 abstract "SummaryThe regulation of photosynthetic carbon metabolism is reviewed, drawing a distinction between regulatability and regulatory capacity. The former describes whether the properties of an enzyme allow its activity to be changed in vivo by naturally occurring mechanisms. The latter describes whether a change in the activity of an enzyme will lead to a change in flux through the pathway.The regulatory properties of the enzymes of photosynthetic carbon metabolism are first reviewed. I argue that they allow us to understand how a coordinated change in flux is achieved in response to changes in the external conditions, or the physiological status of the plant. This is illustrated by the responses of the Calvin cycle to irradiance, and by the regulation of partitioning and its coordination with the operation of the Calvin cycle. Important areas requiring more research are indicated, in particular the fine regulation of Rubisco and its activation state, regulation of sucrose phosphate synthase, and gene expression in mature leaves in response to external and physiological signals.I then discuss how regulatory capacity can be assessed by measuring flux control coefficients. The flux control coefficients for the processes involved in CO2 entry into the leaf, Rubisco, several Calvin cycle enzymes, four enzymes in the pathway of starch synthesis, and several enzymes for sucrose synthesis are estimated. Control is usually shared, and the distribution depends on short-term and long-term conditions. It is pointed out that acclimation can lead to an adjustment in the balance between different processes, which allows a leaf to escape from a one-sided limitation. Unexpectedly, many enzymes that catalyze readily reversible reactions develop substantial flux control coefficients when they are decreased two- to three-fold below wildtype levels. On the other hand, many enzymes that catalyze irreversible reactions and possess regulatory properties do not exhibit control in the wildtype, and will not until a large decrease in enzymic amount is engineered.The consequences for our understanding of pathway structure and regulation are then discussed, in particular the influence of the kinetic properties of an enzyme on its flux control coefficients. It is shown by theoretical analysis and experimental examples that the flux control coefficients will usually increase when the sensitivity of an enzyme to regulatory effectors is decreased. As a consequence, most enzymes with high regulatability actually have a low regulatory capacity! Their sophisticated regulatory properties serve to coordinate their activity with the rest of the pathway, and allow their activity to respond to changes initiated elsewhere, but they do not themselves provide a site from which flux can be altered. Enzymes with a high regulatory capacity possess kinetic properties which allow them to be modulated via effectors that do not interact strongly with the rest of the pathway. Processes with no regulatability also sometimes exert control. It is proposed that selection to avoid the consequences of metabolic imbalance and inefficient allocation tend to prevent individual processes from exerting total control." @default.
• W2110777712 created "2016-06-24" @default.
• W2110777712 creator A5066445561 @default.
• W2110777712 date "1996-01-01" @default.
• W2110777712 modified "2023-10-10" @default.
• W2110777712 title "Metabolic Regulation of Photosynthesis" @default.
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