The Calvin-Benson-Bassham CBB Cycle

C02 (and oxygen see below) adds to C2 of this molecule in its enediol form. Basically, only two key enzymes are required in addition to the enzymatic outfit of most cells, namely the carboxylase ribulose 1,5-bisphosphate carboxylase/oxy- 

The key CBB cycle enzyme, RubisCO, is the most abundant protein in the world [8], as it can comprise up to 50% of the total soluble protein in the chloro-plasts or in bacteria using this cycle. This fact is a consequence of the notorious catalytic inefficiency of RubisCO, that is, a low affinity for C02, a slow catalytic turnover rate, and a wasteful oxygenase side reaction responsible for photorespiration, resulting in a futile cleavage of the substrate to form phosphoglycolate as a side product. However, the CBB cycle enzymes are oxygen-insensitive and can easily be controlled, because the whole pathway is separated from 

Pathway Amount of ATP for synthesis of one triose phosphate Reductants for synthesis of one triose phosphate C02-fixing enzymes Active C02 species C02 fixation products which may be used for biosynthesis Key enzymes 

Calvin-Benson-Bassham cycle (reductive pentose phosphate cycle) 9 6 NAD(P)H RubisCO C02 3 - Phosphogly cerate RubisCO Phosphoribulokinase 

Reductive citric acid cycle 5 3 NAD(P)H, 1 unknown donor , 2 ferredoxin 2-Oxoglutarate synthase Isocitrate dehydrogenase6 Pyruvate synthase PEP carboxylase C02 C02 C02 HCOJ Acetyl-CoA, pyruvate, PEP, oxaloacetate, succinyl-CoA, 2-oxoglutarate 2-Oxoglutarate synthase, ATP-citrate lyase 

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Figure 3.1 The Calvin-Benson-Bassham (CBB) cycle. , phosphoribulokinase , ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO).

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