Journal
FUNCTIONAL PLANT BIOLOGY
Volume 33, Issue 10, Pages 921-929Publisher
CSIRO PUBLISHING
DOI: 10.1071/FP06059
Keywords
fallover; heat stress; high temperature; Rubisco
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High temperature inhibits photosynthesis by several mechanisms including reduction in Rubisco activity. While the initial reaction velocity of purified, fully carbamylated, inhibitor-free Rubisco increases with temperature in vitro, over time, the reaction velocity slowly declines (fallover) because of the enzymatic and non-enzymatic production of inhibitors from the substrate ribulose-1,5-bisphosphate. We tested whether fallover could contribute to the decline in Rubisco activity observed in leaf extracts at high temperature. Production of D-xylulose-1,5-bisphosphate (XuBP), an inhibitor of Rubisco, was greater at 35 and 45 degrees C than at 25 degrees C but fallover was less severe at 35 and 45 degrees C than at 25 degrees C, both in rate and extent under saturating CO2 and ambient O-2. This apparent dichotomy is consistent with the catalytic site of Rubisco loosening at higher temperatures and releasing inhibitors more easily. The loosening of the catalytic site was supported by the observation that RuBP and XuBP were released from their complexes with uncarbamylated, Mg2+-free Rubisco faster at 35 and 45 degrees C than at 25 degrees C. We conclude that, although XuBP production increased relative to catalytic throughput at higher temperatures, this was more than compensated for by its faster release, resulting in less fallover inhibition at higher temperatures.
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