Rubisco then attaches the oxygen to the sugar chain, forming a faulty oxygenated product. The plant cell must then perform a costly series of salvage reactions to correct the mistake. Plants and algae build a large, complex form of rubisco, composed of eight copies of a large protein chain and eight copies of a smaller chain. The protein in the PDB entry 1rcx is taken from spinach leaves. 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. 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.