REFINEMENT OF THE STRUCTURE OF HUMAN BASIC FIBROBLAST GROWTH-FACTOR AT 1.6 ANGSTROM RESOLUTION AND ANALYSIS OF PRESUMED HEPARIN-BINDING SITES BY SELENATE SUBSTITUTION
Ae. Eriksson et al., REFINEMENT OF THE STRUCTURE OF HUMAN BASIC FIBROBLAST GROWTH-FACTOR AT 1.6 ANGSTROM RESOLUTION AND ANALYSIS OF PRESUMED HEPARIN-BINDING SITES BY SELENATE SUBSTITUTION, Protein science, 2(8), 1993, pp. 1274-1284
The three-dimensional structure of human basic fibroblast growth facto
r has been refined to a crystallographic residual of 16.1% at 1.6 angs
trom resolution. The structure has a Kunitz-type fold and is composed
of 12 antiparallel beta-strands, 6 of which form a beta-barrel. One bo
und sulfate ion has been identified in the model, hydrogen bonded to t
he side chains of Asn 27, Arg 120, and Lys 125. The side chain of Arg
120 has two conformations, both of which permit hydrogen bonds to the
sulfate. This sulfate binding site has been suggested as the binding s
ite for heparin (Eriksson, A.E., Cousens, L.S., Weaver, L.H., & Matthe
ws, B.W., 1991, Proc. Natl. Acad. Sci. USA 88, 3441-3445). Two beta-me
rcaptoethanol (BME) molecules are also included in the model, each for
ming a disulfide bond to the S(gamma) atoms of Cys 69 and Cys 92, resp
ectively. The side chain of Cys 92 has two conformations of which only
one can bind BME. Therefore the BME molecule is half occupied at this
site. The locations of possible sulfate binding sites on the protein
were examined by replacing the ammonium sulfate in the crystallization
medium with ammonium selenate. Diffraction data were measured to 2.2
angstrom resolution and the structure refined to an R-factor of 13.8%.
The binding of the more electron-dense selenate ion was identified at
two positions. One position was identical to the sulfate binding site
identified previously. The second selenate binding site, which is of
lower occupancy, is situated 5.6 angstrom from the first. This ion is
hydrogen bonded by the side chain of Lys 135 and Arg 120. Thus the sid
e chain of Arg 120 binds two selenate ions simultaneously. It is sugge
sted that the observed second selenate binding site should also be con
sidered as a possible binding site for heparin, or that both selenate
binding sites might simultaneously contribute to the binding of hepari
n.