ch 7 Flashcards
(50 cards)
what is hemoglobin?
oxygen transport protein found in red blood cells; carreis oxygen from the lungs to the tissues and simultaneously transports carbon dioxide and hydrogen ions back to the lungs
what is myoglobin
oxygen storage protein; found in muscle, provides a reserve supply of oxygen in times of need
myoglobin structure
alpha helices that form globular structure
2 forms of myoglobin
deoxymyloglobin, oxymioglobin
what does ability of myoglobin and hemoglobin to bind o2 depend on?
bound prostetic group called heme
what gives blood and muscle distinctive color
heme group
structure of heme group
organic component (protoporphyrin) and a central iron atom bonded to four pyrrole nitrogen atoms
under normal conditions, what charged state is the iron in?
Fe2+ which can form an additional 2 bonds one on each side of heme plane (5th and 6th coordination sites)
what is the 5th coordination site of the heme group occupied by
imidazole ring of a histidine residue from a protein (called the proximal histidine)
what is the sixth coordination site used for?
binding to oxygen
how does binding of oxygen affect the heme group?
changes the position of iron which goes into the plane when oxygen is present
what happens when oxygen binds?
partial transfer of an electron from the ferrous ion to oxygen. this is a complex between Fe3+ ion and a superoxide ion
why is the superoxide ion sometimes damaging to living cells
if superoxide leaves, heme group containging Fe3+ cannot bind to oxygen again
how does myoglobin prevent superoxide from being released
binding pocket of myoglobin has additional histidine called distal histidine which hydrogen bonds to the bound oxygen
structure of hemoglobin
4 chains: 2 identical alpha chains and 2 identical beta chains . each subunit has similar structure to myoglobin
hemoglobin binds oxygen very efficiently (how much of carrying capacity?) and how is this compared to myoglobin
90 percent. myoglobin can only use 7percent under similar conditions
four chains in hemoglobin bind oxygen how?
cooperatively meaning the binding of oxygen in 1 site increases chances for remaning sites to bind o2
how does cooperativity happen?
hem globin undergoes large changes in quarternary structure alpha1beta1 and alpha2beta2 dimers rotate 15 degrees with respect to each other
what is the t state
quaternary structure observed in the deoxy form. tense state. quite rigid
what is the r state
relaxed state. quarternary structure in fully oxygenated form. relaex because more flexible to undergo structural changes
which state r or t binds o2 better?
r state binds o2 with higer afinity.
what are the 2 models of cooperativity?
concerted model
- hemoglobin molecules only exist in the R or T states
Sequential model
- hemoglobin can exist in partial R or T states
- at each state of oxygen loading, an equlibrium exists between 2 states, more oxygen shits equilibrium towards R state more.
there can be a mixture of R and T states within a hemoglobin tetramer
- binding of ligand changes conformation of subunit to which it binds which induces neighboring subunits to increase affinity for ligand.
how does hemoglobin behavior described by both forms of cooperativity?
follows concerted model when three out of 4 sites are bound to o2. quaternary structure consistent with the R state and the o2 affinity of the remaining site is 20 fold greater than completely deoxygenated hemoglobin
follows sequential model when one of the four sites is bound with o2. has quaternary stuructre assoicated with the tstate and the empty sites only bind o2 with 3 times greater affinity than deoxygenated hemoglobin
what happens to the sigmoidal curve when all the molecules are in the t state? when the fraction of molecules in r state increases? when all of the sites within r state molecules are filled?
The sigmoidal curve is shallow at low oxygen concentrations when
all of the molecules are in the T state
• Curve becomes steeper as fraction of molecules in R state
increases
• Flattens when all of the sites within R state molecules are filled
