MAXIMUM LIKELIHOOD DENSITY MODIFICATION BY PATTERN RECOGNITION OF STRUCTURAL MOTIFS

摘要

An electron density for a crystallographic structure having protein regions and solvent regions is improved by maximizing the log likelihood of a set of structures factors {Fh} using a local log-likelihood function: LL(ρ(x,{Fh})) = 1n[p(ρ(x| PROT)pPROT(x) + p(ρ(x)]| SOLV)pSOLV(x) + p(ρ(x)| H)pH(x)], where |PROT(x) is the probability that x is in the protein regions, p(ρ(x)|PROT) is the conditional probability for ρ(x) given that x is in the protein region, and pSOLV(x) and p(ρ(x)|SOLV) are the corresponding quantities for the solvent region, pH(x) refers to the probability that there is a structural motif at a known location, with a known orientation, in the vicinity of the point x; andp (ρ(x)|H) is the probability distribution for electron density at this point given that the structural motif actually is present. One appropriate structural motif is a helical structure within the crystallofraphic structure.