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# Photosynthesis: RuBisCO General Properties

BIOCHM-YK6TU2

The photosynthetic enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase oxygenase) is one of the most important and abundant proteins on earth. This dual function enzyme first evolved under anaerobic conditions. In land plants, the enzyme consists of 8 large subunits and 8 small subunits with a molecular mass of about 540 KDa.

Use your understanding of photosynthesis and Rubisco to determine which of the following statements is UNTRUE regarding RuBisCO.

A

The RuBisCO protein is very abundant in plant leaves largely because the enzyme is extremely inefficient in land plants.

B

The dual function of RuBisCO refers to the enzyme catalyzing both ${ CO }_{ 2 }$ fixation and ${ O }_{ 2 }$ fixation, with both gases competing for the same binding site.

C

In land plants, the RuBisCO large subunit is encoded by the chloroplast genome, whereas the small subunit is nuclear encoded.

D

RuBisCO is the primary carboxylase in both ${ C }_{ 3 }$ and ${ C }_{ 4 }$ plants, whereas phosphoenolpyruvate (PEP) carboxylase is the active carboxylase in Crassulacean acid metabolism (CAM) plants (e.g., cacti).

E

RuBisCO is only functional in the light and is a rate-limiting reaction of the Calvin-Benson-Bassham ${ CO }_{ 2 }$ fixation pathway.