Virtuelle (Nano) Welten: PDB Molecule of the Month 11: Rubisco

Sixteen Chains in One

Plants and algae build a large, complex form of rubisco (shown on the left), composed of eight copies of a large protein chain (shown in orange and yellow) and eight copies of a smaller chain (shown in blue and purple). The protein shown here is taken from spinach leaves (coordinates may be found in the PDB entry 1rcx; the tobacco enzyme may be found in 1rlc). Many enzymes form similar symmetrical complexes. Often, the interactions between the different chains are used to regulate the activity of the enzyme in the process known as allostery. Rubisco, however, seems to be rigid as a rock, with each of the active sites acting independently of one another. In fact, photosynthetic bacteria build a smaller rubisco (shown on the right, taken from PDB entry 9rub) composed of only two chains, which performs its catalytic task just as well. So, why do plants build a large complex? The answer might lie in the crowded conditions under which rubisco performs its job. By packing many chains together into a tight complex, the protein reduces the surface that must be wetted by the surrounding water. This allows more protein chains, and thus more active sites, to be packed into the same space.

Rubisco (Ribulose Bisphosphate Carboxylase/Oxygenase)

Sixteen Chains in One