Many other proteins lead double lives, performing two entirely different functions. Three examples are shown here. The enzyme retinal dehydrogenase, which converts light- sensing retinal into the regulatory molecule retinoic acid, is shown on the left (PDB entry 1o9j
), with the cofactor NADH in green. Its second job is to modify the consistency and absorbance of eye lenses, where it is called eta-crystallin
. Cytochrome c
(PDB entry 3cyt
) is shown at upper right in red. It performs a familiar role in energy production, ferrying electrons in the mitochondria. But when the cell is damaged, cytochrome c spills out into the cytoplasm and performs its second job as part of the apoptosome
: it starts a cascade of responses that ultimately lead to programmed cell death (apoptosis). The third moonlighting protein shown here is phosphoglucose isomerase (PDB entry 2pgi
), one of the ten enzymes that perform glycolysis
. It is also secreted outside cells, where it acts as a cellular messenger with several names: neuroleukin, autocrine mobility factor, and differentiation and maturation mediator. The names give you an idea of the specific messages it carries from cell to cell.