New insights into allosteric mechanisms from trapping unstable protein conformations in silica gels.
To understand why the classical two-state allosteric model of Monod, Wyman, and Changeux explains cooperative oxygen binding by hemoglobin but does not explain changes in oxygen affinity by allosteric inhibitors, we have investigated the kinetic properties of unstable conformations transiently trapped by encapsulation in silica gels. Conformational trapping reveals that after nanosecond photodissociation of carbon monoxide a large fraction of the subunits of the T quaternary structure has kinetic properties almost identical to those of subunits of the R quaternary structure. Addition of allosteric inhibitors reduces both the fraction of R-like subunits and the oxygen affinity of the T quaternary structure. These kinetic and equilibrium results are readily explained by a recently proposed generalization of the Monod-Wyman-Changeux model in which a pre-equilibrium between two functionally different tertiary, rather than quaternary, conformations plays the central role.
Viappiani
Cristiano
C
Department of Physics, University of Parma, 43100 Parma, Italy.
Bettati
Stefano
S
Bruno
Stefano
S
Ronda
Luca
L
Abbruzzetti
Stefania
S
Mozzarelli
Andrea
A
Eaton
William A
WA
D000494:Allosteric Regulation; D006454:Hemoglobins / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008958:Models, Molecular; D010100:Oxygen / Q000378:metabolism; D010108:Oxyhemoglobins / Q000737:chemistry; D011487:Protein Conformation; D011506:Proteins / Q000737:chemistry*; D058428:Silica Gel; D012822:Silicon Dioxide; D013053:Spectrophotometry
D006454:Hemoglobins; D010108:Oxyhemoglobins; D011506:Proteins; D058428:Silica Gel; D012822:Silicon Dioxide; C100003:deoxyhemoglobin; D010100:Oxygen