Fig. 6.

Suppression of futile cycles is especially important in low irradiance. Simultaneous activity of CBC enzymes and respiratory enzymes will lead to futile cycles that would consume ATP and decrease photosynthetic efficiency. To catalyse the high CBC flux that is attained in high light, the activities of CBC enzymes are much higher than those of respiratory enzymes. In the absence of regulation, the rate of futile cycling is therefore likely to be set by the capacity of the respiratory enzymes (schematically illustrated as the dark red line) and in low irradiance may even exceed the energy available from respiration and photosynthetic electron transport (black line). Inactivation of CBC enzymes in darkness and low irradiance (red line) will reduce wasteful futile cycles in low light. Inactivation of respiratory enzymes in the light (yellow line) would further reduce futile cycles in low light and also supress futile cycling in high light when CBC enzymes are activated.