Infrared crystallographic investigation of T-state hemoglobin.
Polarized IR measurements on single crystals of human hemoglobin, grown under low salt conditions that stabilize the T quaternary structure, allow spectral features to be associated with individual sites within the molecule. Differences between the a- and c-polarized IR spectra in the sulfhydryl stretching region distinguish the contributions of individual Cys residues to the S-H band and lead to an evaluation of possible H-bonding partners for Cys beta-112. Successful modelling of both crystal and solution S-H spectra with component bands having identical frequencies and bandwidths, supports the use of the X-ray structure as a model for the low affinity T-state in solution. Polarization analysis of a crystal partially saturated with CO reveals comparable occupancy of the alpha- and beta-hemes. In the T-state crystal, the C-O bands are broader and lower in frequency than in the R-state solution, and 20% of the CO-ligated beta-subunits adopt an alternate conformation with a 1967 cm(-1) C-O frequency. The latter observation reflects an energetically significant disruption of the distal heme pocket upon CO binding to beta-hemes in the low affinity T-state.
Khachfe
H
H
Department of Physics and Center for Interdisciplinary Research on Complex Systems, Northeastern University, Boston, MA 02115, USA.
Mylrajan
M
M
Sage
J T
JT
D000328:Adult; D003461:Crystallography; D006454:Hemoglobins / Q000737:chemistry*; D006801:Humans; D011487:Protein Conformation; D012996:Solutions; D013055:Spectrophotometry, Infrared / Q000379:methods*
D006454:Hemoglobins; D012996:Solutions; C100003:deoxyhemoglobin