Blue copper proteins as honorary cytochromes: The structure and evolution of blue copper proteins

The blue copper proteins are among the most beautiful macromolecules that w e know, and the intensity of the colour of azurin per copper atom is eighty times as that of the cuprammonium ion. They occur in the periplasm of Gram positive bacteria and in the photosystems of blue green algae (cyanobacter ia) and in algal and higher plant chloroplasts. The function of these prote ins in electron transport is very similar to those of the soluble cytochrom es C-6 ("algal cytochrome f") and c(6) ("Pseudomonas cytochrome c-551"), so much so that one thinks of them as 'honorary cytochromes'. Several differe nt sequence classes have been recognized, including azurin, plastocyanin, a micyanin, pseudoazurin and rusticyanin. There is considerable three-dimensi onal similarity in the copper binding sites of the proteins, and have as li gands two histidine, one cysteine and one methionine side chains. The proteins have been co-opted to function in a wide range of metabolisms, and azurin is often (but not always) associated with denitrification. The gene has been adapted to produce an outer membrane protein in gonococci, wh ile azurin, amicyanin and pseudoazurin are involved in methylotrophy in dif ferent organisms - though in other closely;related bacteria, cytochromes c( 8) play equally versatile roles. While the overall structures of each of the proteins are very similar where ver they are found, when duplicate genes occur in the same organism, the di fference between the isoforms are large, even if no functional difference b etween the forms has been detected in vitro. The time is ripe and the techniques now available for the function and rela tionships of these proteins to be elucidated genetically, which has not bee n possible before because of the multiplicity of alternative pathways in ba cterial electron transport systems.