Proteins in certain organisms have the ability to bind the molecule carbon monoxide. One of these proteins is the CooA protein, which exists in similar forms in two organisms: C. hydrogenoformans and R. rubrum. The R. rubrum CooA protein is very selective for carbon monoxide only, possibly due to the presence of a cysteine amino acid at position 75 in the structure of the protein. The CooA from C. hydrogenoformans does not have a cysteine amino acid in this position and binds more promiscuously, binding nitric oxide as well as carbon monoxide, for example. In order to test the hypothesis that the cysteine amino acid at position 75 causes the high selectivity of the R. rubrum CooA towards carbon monoxide, we expressed and purified a mutated CooA protein from R. rubrum in which the cysteine at position 75 was mutated to a serine amino acid. If the purification is successful, the mutant R. rubrum CooA can then be tested to see if it now binds nitric oxide as well as carbon monoxide, like the C. hydrogenoformans CooA protein
