Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
Regulation of Dark Reactions

The rate-limiting step that determines the speed of dark reactions is the fixation of СО2 in a reaction catalyzed by ribulose diphosphate carboxylase, which yields 3-phosphoglycerate. Ribulose diphosphate carboxylase, whose molecule consists of 16 subunits, is an allosteric enzyme. It can be activated by three distinct changes, each of which results from the illumination of METABOLISM/14.html">Chloroplasts. These changes are as follows:

1. Increase in pH. Upon illumination of chloroplasts, H+ ions move from the stroma into the thylakoids. This leads to an increase in stroma pH and activates the carboxylase located on the outer surface of the thylakoid membrane.

2. Influx of Mg2+ ions into the stroma.

Mg2+ ions enter the stroma when H+ ions shift from the stroma into the thylakoids upon chloroplast illumination.

3. Accumulation of NADPH generated by Photosystem I upon illumination. Thus, although СО2 fixation by ribulose diphosphate carboxylase is part of the dark reactions, it is indirectly stimulated by chloroplast illumination. In the same indirect manner, chloroplast illumination activates several Other Enzymes involved in The Calvin Cycle, as well as ATP synthase.



Last update: 06/08/2026

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