BINDING CONSTANTS FOR A PHYSIOLOGICAL ELECTRON-TRANSFER PROTEIN COMPLEX BETWEEN METHYLAMINE DEHYDROGENASE AND AMICYANIN - EFFECTS OF IONIC-STRENGTH AND BOUND COPPER ON BINDING

Two soluble proteins, methylamine dehydrogenase and amicyanin, form a physiologically relevant complex in which intermolecular electron tran sfer occurs. To characterize and quantitate the binding of these two w eakly-associating proteins, an ultrafiltration binding assay has been developed which involves brief centrifugation of mixtures of proteins in centrifuge concentrators followed by quantitation of proteins on ea ch side of the filtration membrane by HPLC. Under low ionic strength c onditions which are optimal for the redox reaction between these prote ins, a K(d) of 4.5 muM was measured for the methylamine dehydrogenase- amicyanin complex. The K(d) increased by 8-fold in the presence of add ed salt. Apoamicyanin, which is known from crystallographic analysis t o be structurally very similar to amicyanin, exhibited a much higher K (d) and much less specific binding than did the holoprotein. Apoamicya nin also exhibited apparent self-association at low ionic strength whi ch was not observed with amicyanin. These observations are correlated with the known crystal structures of these proteins, free and in compl ex, and with the available biochemical information on the interactions of these two proteins.