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BICH 410 Exam 2
Terms in this set (25)
an intracellular protein found in muscle tissue that reversibly binds oxygen and increases the concentration of such oxygen in respiring tissues in order to generate ATP
-_________________ increases the effective solubility of oxygen in muscle tissue
Myoglobin is composed of a single polypeptide chain of ______ residues in 8 alpha-helices labeled A-H
-the oxygen is bound by myoglobin to a heme which is a porphyrin consisting of 4 pyrrole rings linked by methylene bridges--the pyrrole ring has a lone pair of electrons on the Nitrogen which is required for aromaticity and makes it not basic
In the structure of protoporphyrin, the central iron is in the _____ ferrous state--if the iron does become oxidized into Fe3+ ferric state, it forms metmyoglobin (MetMb) and methemoglobin (MetHb).
-neither of these forms of met bind oxygen, instead the heme coordinates tightly to a water molecule as the 6th ligand
Which enzyme in blood cells converts the Fe3+ oxidized iron center back to the Fe2+ state in which hemoglobin and myoglobin are able to bind oxygen again?
The iron atoms of a heme, either in the ferrous (Fe2+) or ferric (Fe3+) states, prefer to bind 6 ligands in an octahedral geometry.
-4 of the ligands are the nitrogen atoms of the 4 pyrrole rings of the heme
-the 5th ligand comes from _________________ F8
-when oxygen binds to the ferrous (Fe2+) heme, it become the 6th ligand
-water becomes the 6th ligand for the heme if the iron is in a ferric state (Fe3+)
The oxygenated heme of myoglobin is called oxymyoglobin--the oxygen atom bound to the heme is not perpendicular to the plane of the heme and it has a ______ degree angle.
The unoxygenated heme of myoglobin is called deoxymyoglobin--here, the 6th coordination site is vacant.
On the same side of the heme plane that oxygen binds to is ________________ E7. It is too far away to coordinate with the iron of the heme, but can sterically interact with the binding of O2--this makes the oxygen binding site of the heme a sterically hindered region.
Free heme in solution has 25,000 times more affinity for CO than O2. In myoglobin and hemoglobin, CO only binds 250 times greater than oxygen because _____-____ (distal histidine) forces the CO molecule to tilt away from the preferred perpendicular alignment with the plane of the heme.
-this reduction in the affinity of Mb/Hb for CO allows trace amounts of CO to be generated during metabolism which would otherwise occupy Mb and Hb binding sites
-CO is still a potent posion!!
In deoxymyoglobin, the ferrous iron atom (Fe2+) has 5 ligands and lays ______ Angstroms above the plane of the heme towards His-F8. This gives the iron porphyrin a complex dome shape.
In oxymyoglobin, when oxygen binds, the iron atom is pulled back into the plane of the heme such that it is only _____ Angstroms above the porphyrin. The consequences of this are small for myoglobin, but great impacts hemoglobin.
A graph of log(Y/(1-Y)) versus log(pO2) is called a Hill plot. For myoglobin, the Hill plot is linear.
-At half saturation (when Y=0.5), Y/(1-Y) = 1 and the log(1) = 0. At this point, pO2=K=p50.
-the slope of the Hill plot at the point of log(Y/1-Y) = 0, the midpoint of binding is called the Hill Coefficient
-the Hill Coefficient for myoglobin is ____--this means that the oxygen atoms bind independently of each other
Hemoglobin is a compact globular protein. Like myoglobin, it binds oxygen to a heme, but hemoglobin is a ______________--it is composed of 4 polypeptide chains, each structurally similar to the single polypeptide chain of myoglobin. Each subunit contains a heme, allowing hemoglobin to bind 4 oxygen atoms.
-adult hemoglobin is an alpha2beta2-type _______________
The 2 different subunits of Hemoglobin, alpha and beta, are virtually identical to the structure of _______________. The beta-subunit is 7 AA residues shorter than _________________ (153) and its last helix, H, is shorter as well. The alpha-subunit is 12 AA residues shorter and has a shorter H helix and lacks Helix D.
The tetrameric quaternary structure of Hemoglobin is important to its function. When a molecular of oxygen binds to a heme in Hb, the heme iron is drawn into the plane of porphyrin ring. This slight movement sets off a chain of conformational events that are transmitted to adjacent subunits, dramatically _______________ the affinity of the heme for oxygen!
The hemoglobin molecule is highly symmetrical and spherical--the hemes are located in the clefts between the E and F helices and are exposed to the aqueous solvent. The heme groups are far apart--the closed 2 hemes are located between alpha1 and beta2 or alpha2 and beta1 & are separated by _____ Angstroms.
The hemoglobin subunit interactions are primarily between alpha1 and beta1 (or alpha2 and beta2 due to symmetry) interface consisting of 35 residues. These contacts involve helices B, G and H and the GH corner.
-these contacts are extensive and important for subunit packing but remain _________________ when deoxyhemoglobin binds oxygen to become oxyhemoglobin
Hb subunit interactions can also be between the interface of alpha1 and beta2 (or alpha2 and beta1) subunits and involve 19 residues. These contacts are called sliding contacts & involve helices C, G, and the FG corner
-when Hb binds an oxygen, a conformational change occurs such that these sliding contacts are altered & each of the alpha-beta dimers moves as a rigid body so that the 2 halves of the molecule slide past each other upon ________________________ of the heme
Hemoglobin sliding contacts: the two halves of Hb rotate ____ degrees around an imaginary pivot passing through the alpha-beta subunits; some atoms in the sliding contacts interface move as much as 6 Angstroms.
Looking at the ferrous iron (Fe2+) center of the heme where oxygen binds in Hemoglobin, the changes in the Fe2+ geometry upon oxygen binding are identical to those of myoglobin. In deoxyhemoglobin, the ferrous iron atom has 5 ligands and lays _____ Angstroms above the plane of the heme towards His-F8--this gives the porphyrin complex a dome shape.
In oxyhemoglobin, when oxygen binds, the iron atom is pulled back by ____ Angstroms towards the plane of the heme so that it is only 0.2 Angstroms above the porphyrin--the binding of oxygen pulls the Fe2+ into the heme plane.
When oxygen binds to hemoglobin to form oxyhemoglobin, the _____ Angstrom displacement seems trivial, but the biological consequences are far reaching. As the iron moves, it drags His-F8 along with it which causes Helix F and EF corner to follow--these shifts are transmitted across the subunits interfaces where they trigger conformational readjustments.
The quantity, ____, called the Hill constant, increases with the degree of cooperativity of a reaction and provides a way of characterizing the ligand binding reaction.
If n=1, as the case for myoglobin, the hill equation describes a hyperbolic binding curve and the binding is said to be _____________________________.
If n>1, then the reaction is said to be __________________ cooperative. The ligand binding increases the affinity of the protein for further ligand binding.
If n<1, then the reaction is said to be __________________ cooperative. The ligand binding decreases the affinity of the protein for subsequent ligand binding.
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