Optil rotatory dispersion and circular dichroism of human carbonic anhydrases B and C.

Studies of optical rotatory dispersion (ORD) and circular dichroism (CD) are reported on human carbonic anhydrases B and C in the spectral region below 320 mp. Most of the observed CD spectrum of each enzyme could be described in terms of three principal Gaussian bands: a strong negative band near 216 rnp, a weaker negative band near 270 to 275 mp, and a positive band intermediate between the other two in position. Above 260 mp, all CD values are negative; there is clear evidence of tie structure above 280 mp, involving at least two additional CD bands. The ORD spectra above 260 rnp show several peaks and troughs, which are characteristic for each enzyme. The principal troughs at shorter wave lengths lie at 222 rnp for Enzyme B and at 226 rnp for Enzyme C. At still shorter wave lengths, the ORD values rise to low peaks, at 204 rnp, with [m’] = -300 for Enzyme B and +2750 for Enzyme C. These patterns are very different from those characteristic of either a-helical or /3 structures. Asymmetrical interactions involving the aromatic side chains almost certainly make important contributions to ORD and CD at the shorter wave lengths, as they clearly do at the longer wave lengths, above 250 mp. On acid denaturation, the longer wave length Cotton effects vanish, and the ORD and CD spectra alter in such a way as to suggest the presence of 10 to 20% a-helix in the acid-denatured proteins. The changes on exposure to high pH are more complex. Near pH 11, the negative CD band near 270 to 275 rnp becomes less intense and shifts to longer wave lengths; the positive band at 232 rnp in Enzyme B also shifts to longer wave lengths. It is tentatively suggested that these bands arise in part from interactions of tyrosine residues. The