CYTOCHROME STRUCTURES DETERMINED AB INITIO: CYTOCHROME C6 (9.3 KD) AND DIMERIC CYTOCHROME C3 (26 KD)
C. Frazao1, G.M. Sheldrick2 and M.A. Carrondo1
1ITQB, Apartado 127, 2780-Oeiras, Portugal, 1Inst. Anorg. Chem., Uni. Göttingen, Göttingen, Germany.
Until the 1980s one assisted to the development of computer algorithms which were able to derive phases from experimental X-ray measurements, leading eventually to automatic structure determinations for most structures with less than 100 non-hydrogen atoms. More recently the use of cryo conditions and of synchrotron radiation sources has allowed very high resolution data collection, in particular for proteins, which prompted the development of structure resolution approaches making use of the specificities of these (near) atomic resolution data.
Cytochrome c6, an 89aa cytochrome electron carrier within the photossynthetic machinery of green algae Monoraphidium braunii, was solved using Patterson search with hyper-sharpened data ((Eo)1/2 3Fo) (1), followed by a peak list optimization procedure. The dimeric cytochrome c3, with 218aa and 8 hemes, is also an electron carrier, within the respiratory chain of strictly anaerobic bacteria Desulfovibrio gigas. Its structure was determined directly by program SHELXD (2) using the half-baked approach, a real/reciprocal space recycling method for structure determination which uses two loops of iterative procedures: in a first loop, a specified number of trial atoms are eliminated to maximize of the figure of merit SEc2(Eo2-1) alternating with tangent phase refinement and, in a second loop, applied only for solutions with a good correlation coefficient, the peak list optimization technique is used to complete the structure as much as possible.