CHEMICAL MODIFICATION OF ALPHA-SUBUNIT TRYPTOPHAN RESIDUES IN SCHIZOSACCHAROMYCES-POMBE MITOCHONDRIAL-F1 ADENOSINE 5'-TRIPHOSPHATASE - DIFFERENTIAL REACTIVITY AND ROLE IN ACTIVITY

Chemical modification of mitochondrial F1-ATPase from Schizosaccharomyces pombe by the tryptophan-specific reagent N-bromosuccinimide (NBS) at pH 5.0 in the presence of 20% glycerol produced a characteristic lowering in both enzyme absorbance at 280 nm and intrinsic fluorescence at 332 nm that varied with NBS/F1 molar ratio up to a value of 130. Fluorometric titration of tryptophans and correlation to residual ATPase activity showed that modification of three reactive residues among the seven present on alpha- and epsilon-subunits did not markedly modify the enzyme activity but efficiently released endogenous ATP and abolished the fluorescence quenching related to GDP or ATP binding to the catalytic site. Additional modification of one, less reactive, tryptophan altered both negative cooperativity of ATP hydrolysis and sensitivity to azide inhibition and produced a nearly complete inactivation at high NBS/F1 molar ratio. NBS-induced inactivation of F1 was favored by catalytic-site saturation with GDP or low ATP concentration and on the contrary was prevented by noncatalytic-site saturation with ADP or high ATP concentration. When reactive tryptophans were selectively modified by NBS in the presence of ADP, and subunits were isolated after guanidine hydrochloride dissociation by one-step purification on reversed-phase HPLC, the absorbance of alpha-subunit at 280 nm was decreased, whereas that of epsilon-subunit was unchanged. Cyanogen bromide cleavage of alpha-subunit and fragments separation by reversed-phase HPLC showed that one peptide of 3 kDa apparent molecular mass had decreased absorbance. N-Terminal sequencing allowed its identification to fragment 255-282 that contains tryptophan257.