文档介绍：Chapter 6 Proteins in Action
1. Hemoglobin is a multisubunit allosteric protein that carries O2 in erythrocyte.
Hemoglobin is a well-studied and well-understood protein.
It was one of the first proteins to have its molecular mass accurately determined.
The first protein to be characterized by ultracentrifuge.
The first protein to be associated with a specific physiological function.
The first protein with a single amino acid substitution being related to a ic disease (the beginning of molecular medicine).
The first multisubunit protein with its detailed atomic structure determined by X-ray crystallography.
The best understood allosteric protein.
Determination of the atomic structure of hemoglobin A (from normal adult) is very revealing.
The protein molecule exists as a a2b2 tetramer.
Each subunit has a structure strikingly and unexpectedly similar to each other and to that of myoglobin, indicating quite different amino acid sequences can specify very similar 3-D structures.
Extensive interactions exist between the unlike subunits through noncovalent interactions.
Quaternary structure changes markedly when O2 binds. Crystals of deoxyhemoglobin shatter (break) when exposed to O2.
O2 binds to the sixth coordination position of the ferrous iron (as in myoglobin).
Hemoglobin is a much more intricate and sentient (sensitive) molecule than is myoglobin.
The oxygen-binding (dissociation) curve of hemoglobin is sigmoidal and that of myoglobin is hyperbolic.
Myoglobin has a higher affinity for O2, evolved for O2 storage.
Hemoglobin releases O2 efficiently at low oxygen level tissues (evolved for O2 delivery), myoglobin does not.
Oxygen binding of hemoglobin shows positive cooperativity. The binding of O2 (the ligand) at one heme facilitates the binding of O2 at the other hemes on the same tetramer (vice versa, unloading of oxygen at one heme facilitates th