RuBisCO

The Ubiquitous Enzyme

Created by: Joe Harris

June 22, 2005

RuBisCO

General Information• RuBisCo stands for: Ribulose 1,5-bisphosphate carboxylase/oxygenase.

• RuBisCO: is a dual functional enzyme that is directly involved in photosynthesis as well as in photorespiration in plants.

•Its substrate is: Ribulose 1,5-bisphosphate.

RuBisCO

General Information...• In photosynthesis, it catalyzes the initial step in the fixation of carbon dioxide that produces 2, 3-carbon molecules (PGA).

• During photorespiration it catalyzes O2 that produces 1 PGA molecule and a 2 carbon phosphoglycolate which is a competing oxygenation reaction.

• RuBisCO is found in higher plants, photosynthetic bacteria, algae and other bacteria.

RuBisCO

RuBisCO

RuBisCO’s Biochemistry

• RuBisCO is made up of 8 large subunits, and 8 small subunits (hexadecamer), these are referred to as the barrels of the molecule.

• Large subunits usually weigh between 50 to 55 kDa

• Small subunits usually weigh between 12 to 18 kDa

•The large subunits associate as dimers. The N-terminus of one adjacent to the active site of the other.

RuBisCO

RuBisCO

RuBisCO’s Biochemistry…•The catalytic or active sites are present on the large subunits, but are unable to express catalytic activity unless the small subunits are also present.

•The large subunits are more than 80% homologous with each other.

RuBisCO

RuBisCO’s Biochemistry…• The amino acids present in the active sites seem to be conserved and the catalytic properties of the enzyme seems to vary significantly between different organisms.

• In higher plants the enzyme is considerably specific, but highly inefficient. The turnover number for RuBisCO is only 3/sec. The opposite is observed for the Cyanobacteria enzyme.

RuBisCO

RuBisCo’s Biochemistry…• The genes for the large and small subunits occur consecutively on the chromosome and constitute a single operon.

• Large subunit - one gene in each circular DNA molecule of the chloroplasts.

• Small subunit - nuclear, small subunit genes are also multiple and comprise a multigene family of 8 or more members.

RuBisCO

Gene Regulation

• Transcription of the genes for the small subunit is regulated by;

* Environmental signals (light) and endogenous programs regulated by developmental and circadian aspects.

* External CO2 and O2 concentrations

RuBisCO

Regulation• The activity of RuBisCO is regulated by another enzyme known as RuBisCO activase.

• There is a region in the amino acid sequence of RuBisCO activase that interacts with RuBisCO during the regulation process.

RuBisCO

Regulation...• For RuBisCO to become catalytically active, it must be activated by carbamylation of the active site LYS residue which completes the binding for the fundamental Mg++ ion.

• The substrate and inhibitor levels are independent of RuBisCo because the enzyme exists in multiple forms.

RuBisCO

Regulation...• Inhibitors:

* Sugar phosphates

° Ribulose 1,5-bisphosphate (RuBP)

° 2-Carboxyarabinitol 1-Phosphate (CA1P)

°Xylulose 1,5-bisphosphate (XuBP, epimer of RuBP)

° PGA (phosphoglyceraldehyde)

RuBisCO

Regulation...• Activator:

° RuBisCO activase (inhibitor alleviator)

• Other factors (indirectly)

° ATP/ADP, Pi

° pH (transthylakoid)

° CO2, O2, temperature

Any Questions?

Thank You,

The End

Credits:

• Kellogg, Elizabeth A., Nickolas D. Juliano 1997. The Structure and Function of RuBisCO and Their Implications for Systematic Studies. American Journal of Botany 84(3): 413-428.

• Jiang, Cai-Zhong, Steven R. Rodermel, and Richard M Snibles 1993. Photosynthesis, Rubisco Activity and Amount, and Their Regulation by Transcription in Senescing Soybean Leaves. Plant Physiology (1993) 101: 105-112.

• Karp, Gerald 3rd ed. 2003. Cell and Molecular Biology John Wiley & Sons, Inc. (text book).