Kinetic barriers in cytochrome folding

he coirtlently bound hoirie prosthetic group provide.-, <ytoi.hronie r with uniipH' optical and struct ural properties. An i m purl a tit aspect ot licit tc cytochrome i inleractiuii is the coordination of two amino acid residues lu Ilic he.me iron, hi t lie native con forma t ion His IX and Met 80 are coordinal cd In the lu'iiic iron. Upon unfolding. Met-80 is displaced by a second liisti dine residue, for example eitliei His 33 or Hi-39 in ye,-,! iso-'2 cMorliroine < The presence of non-native Hi.s-lieine ligamls (His-33 or His-39) slows folding by a factor of ten compared to a cylorhronie < mutant protein lacking His :i;i and His 39 (II;i:iN.H:ï9K ibü-Ü). The involvenienl of non-native flis liemr iiands in (yiorhrorne r folding has been investigated further by nieasiinuu, the folding and unfolding kinetics of the fl:i;i.\.H39K double mutant, and the two single mutants larking either His-33 (HXiX iso-'J or His-3'J (H39K i> 2). For H33N.!l:39K Iso 2 the folding and unfolding rates are faster than iso-2 throughout the entire giianidino equilibrium transition region. H39K iso-'1 c hibits folding and iiiifoUling rates similar to iso-2. The folding rate-, tor He. iso 2 are similar to Uo 2 but the unfohling rates are significant!) faster than IM> 2. '['lies1 results suggest that the presence of a single iiistidine other His is is sufficient to pioduce a kinetic barrier to the folding of cytochroinc c.