Zinc Finger Motif

Zinc atom stabilizes the structure/fold by coordination bonds

Each motif contains about 30-50 amino acids

Contains regular patterns of Cys / His, which coordinate with Zinc atom

Contains one helix and two small anti-parallel beta sheets in each finger

Zinc holds helix and beta-sheets together

Helix binds to DNA

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#Unit 6. System Physiology – Plant

Chaperonins GroEL and GroES sequester and ensure the correct folding of the newly synthesized large subunit (L). 

The assembly chaperone (RbcX)2 binds the large subunit and provides positional information for proper association of large subunit dimers to form the L8–((RbcX)2)8 complex. 

The small subunit (S) associates with L8–((RbcX)2)8, triggering a conformational change in RbcL and eliciting 

the release of (RbcX)2 and formation of the catalytically mature Rubisco holoenzyme [L]8–[S]8.

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#Unit 6. System Physiology – Plant

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#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.

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#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.

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#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

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#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.