by PhilipJ on 2 January 2008
Molecular processes occur so fast that is it difficult to imagine a 24-hour clock that works at the molecular level. But surprisingly, different organisms have evolved many different ways of doing this. Animal cells use a complex collection of proteins (with fanciful names like Clock, Cryptochrome, and Period) that are rhythmically synthesized and degraded each day. The 24-hour oscillation of the levels of these proteins is controlled by a series of interconnected feedback loops, where the levels of the proteins precisely regulates their own production. A much simpler system has been discovered in cyanobacteria. It is composed of three proteins, KaiA, KaiB and KaiC, that together form a circadian clock. At the beginning of the cycle, KaiA (at the top, PDB entry 1r8j) stimulates the large KaiC hexamer (center, PDB entry 2gbl ), which then adds phosphate groups to itself. Then, as KaiC fills itself up with phosphates, it binds to KaiB (bottom, PDB entry 1r5p ), which inactivates KaiA and allows the phosphates to be slowly removed. As the number of phosphates drops, KaiB falls off and KaiA can start the cycle again.
See the rest at David Goodsell’s page, here.