In fig, Phi is fixed at 0º. Clashes occur at ALL Psi angles. Phi = 0º is physically

impossible.

TLS Online TPP Program

#Id: 7268

#Unit 6. System Physiology – Plant

One way in which rubisco is activated involves the formation of a carbamate– Mg2+ complex on the ε-amino group of a lysine within the active site of the enzyme. Two protons are released. Activation is enhanced by the increase in Mg2+ concentration and higher pH (low H+ concentration) that result from illumination. The CO2 involved in the carbamate–Mg2+ reaction is not the same as the CO2 involved in the carboxylation of ribulose-1,5-bisphosphate.

TLS Online TPP Program

#Id: 7269

#Unit 6. System Physiology – Plant

The activity of rubisco is also regulated by light, but the enzyme itself does not respond to thioredoxin.

Two protons are released during the formation of the ternary complex rubisco–CO2–Mg2+, so activation is promoted by an increase in both pH and Mg2+ concentration.

TLS Online TPP Program

#Id: 7270

#Unit 6. System Physiology – Plant

The binding of sugar phosphates, such as ribulose-1,5-bisphosphate, to rubisco prevents carbamylation. The sugar phosphates can be removed by the enzyme rubisco activase, in a reaction that requires ATP. The primary role of rubisco activase is to accelerate the release of bound sugar phosphates, thus preparing rubisco for carbamylation

TLS Online TPP Program

#Id: 7271

#Unit 6. System Physiology – Plant

Rubisco is also regulated by a natural sugar phosphate, carboxyarabinitol-1-phosphate, that closely resembles the six-carbon transition intermediate of the carboxylation reaction. This inhibitor is present at low concentrations in leaves of many species and at high concentrations in leaves of legumes such as soybean and bean. 

Carboxyarabinitol- 1-phosphate binds to rubisco at night, and it is removed by the action of rubisco activase in the morning, when photon flux density increases.

TLS Online TPP Program

#Id: 7272

#Unit 6. System Physiology – Plant

TLS Online TPP Program

#Id: 7273

#Unit 6. System Physiology – Plant

Upon illumination, the flow of protons from the stroma into the thylakoid lumen is coupled to the release of Mg2+ from the intra thylakoid space to the stroma. These light actuated ion fluxes decrease the stromal concentration of protons (the pH increases from 7 to 8) and increase that of Mg2+ by 2 to 5 mM.