Abstract
Fructose 6-phosphate is an intermediate in the Calvin-Benson cycle and can be acted on by phosphoglucoisomerase to make glucose 6-phosphate (G6P) for starch synthesis. A high concentration of G6P is favorable for starch synthesis but can also stimulate G6P dehydrogenase initiating the glucose-6-phosphate shunt an alternative pathway around the Calvin-Benson cycle. A low concentration of glucose 6-phosphate will limit this futile cycle. In order to understand the biochemical regulation of plastidic glucose 6-phosphate supply and consumption, we characterized biochemical parameters of two key enzymes, phosphoglucoisomerase (PGI) and G6P dehydrogenase (G6PDH). We have found that the plastidic PGI in has a higher Km for G6P compared to that for fructose 6-phosphate. The Km of G6PDH isoform 1 is increased under reducing conditions. The other two isoforms exhibit less redox regulation; isoform 2 is the most inhibited by NADPH. Our results support the conclusion that PGI restricts stromal G6P synthesis limiting futile cycling via G6PDH. It also acts like a one-way valve, allowing carbon to leave the Calvin-Benson cycle but not reenter. We found flexible redox regulation of G6PDH that could regulate the glucose-6-phosphate shunt.
Highlight Glucose 6-phosphate stimulates glucose-6-phosphate dehdrogenase. This enzyme is less active during the day but retains significant activity that is very sensitive to the concentration of giucose6-phopshate.
- Abbreviations
- 6PG
- 6-phosphogluconic acid
- At
- Arabidopsis thaliana
- DHAP
- dihydroxacetone phosphate
- E4P
- erythrose 4-phosphate
- F6P
- fructose 6-phosphate
- FBP
- fructose 1,6-bisphosphate
- G6P
- glucose 6-phosphate
- G6PDH
- glucose-6-phosphate dehydrogenase
- PGA
- 3-phosphoglyceric acid
- PGI
- phosphoglucoisomerase
- PGM
- phosphoglucomutase
- So
- Spinacia oleracea
- Xu5P
- xylulose 5-phosphate