Target enzymes for improved photosynthesis

In recent years, huge efforts have been made in identifying the enzymes that limit photosynthesis. Enzymes that exert a high level of control over fluxes can be used as engineering targets to enhance photosynthesis.

Yet, while over-expression of some target enzymes, such as Sedoheptulose-1,7-bisphosphatase (SBPase), have been shown to enhance photosynthesis in vitro, both over– and under-expression of the target enzyme transketolase decreased photosynthetic rate.

A new paper by Honglong Zhao, Qiming Tang, Tiangen Chang, Yi Xiao and Xin-Guang Zhu of the University of Chinese Academy of Sciences and University of Illinois published in in silico Plants uncovers the origin of these unexpected results.

“We wanted to examine how the Calvin-Benson cycle (CBC) fluxes were affected by enzyme activity and intermediate concentration in the CBC, and to determine why the manipulation of some enzymes in this cycle have no effect on photosynthetic rate,” said Zhao.

The authors determined the impact of positive or negative fold change of enzyme concentration on net photosynthetic CO₂ uptake rates using a dynamic systems model of photosynthetic carbon metabolism.

They found that for some enzymes, photosynthetic CO₂ uptake rate increased with enzyme activity, then plateaued.  For other enzymes, photosynthetic CO₂ uptake rate first increased with enzyme activity, then decreased. Zhao explains, “we were surprised to find that some enzymes generated biphasic responses. By analyzing the location of biphasic enzymes in the CBC and the relationship between enzyme and intermediates for each enzymatic step, we were able to determine that these enzymes always catalyze reactions in a sub-cycle consuming one of the shared substrates and the product is used as a substrate for a reaction of the next sub-cycle.”

The capacity of a biphasic enzyme must be maintained within a narrow range to gain an efficient CO₂ fixation flux. When the capacity of a biphasic enzymes is in excess, the shared substrate is predominantly used in one sub-cycle and becomes a limiting for reactions in another sub-cycle. As a result, the increase in enzymatic capacity decreases, rather than increases, the flux of the CBC. The authors conclude that for this reason, these enzymes are not ideal targets for genetic improvement of photosynthesis.