lecture 10 Flashcards
Myoglobin and Hemoglobin:Protein Structure, Function and Allostery
what are normal blood values for hemoglobin? what happens if there is a deviation to these normal limits?
for men: reference range is 13.5 to 17.5 g/dL
for women: reference range is 12.0 to 15.5 g/dL
iron deficiency anemia occurs when the Hb range is lower than this, maybe due to blood loss, insufficient dietary intake, insufficient absorption
what are some symptoms of iron deficiency anemia? how can we prevent this?
fatigue, weakness, short of breath, exercise intolerance - the body needs oxygen to be able to produce the energy the body needs to carry out activity; diet high in foods containing iron, iron supplementation
why are proteins called hemeproteins/hemoproteins?
because they contain a heme prosthetic group
describe the structure of the heme?
an iron porphyrin structure, large, aromatic tetrapyrrole with an iron atom in the middle and various groups pointing from the core
the iron atoms in the Fe2+ oxidation state allows for reversible O2 binding, T/F?
T, but the local geometry of the Fe2+ atom has a substantial influence on this binding
name examples of hemeproteins?
myoglobin (Mb) and hemoglobin (Hb) are globular proteins
what do named examples of hemoproteins do?
serve as oxygen binding and transport proteins
what is the difference between in functionality between hemoglobin and myoglobin?
hemoglobin is found exclusively in the red blood cells and circulates in blood; myoglobin is found in cardiac and skeletal muscle
myoglobin and hemoglobin are not structurally similar? T/F
F, they are structurally similar
In terms myoglobin and hemoglobin tetramers, how do they work?
so myoglobin is a single chain and works alone, whereas Hb is a tetramer of 2 alpha and 2 beta chains and cooperates, though it has four separate parts and had weak interactions at the contact interfaces, each of the four proteins influence each other and work together
so we know the function of hemoglobin, how does the function work?
so to reiterate, the Hb must bind oxygen in lungs and release it in the capillaries. When a first oxygen binds to Fe in the heme, the heme Fe is drawn into the plane of the porphyrin ring and initiates a series of conformational changes that are transmitted to adjacent subunits, ALLOSTERIC interactions occur, and events at one site has effects on sites far away
what amino acid connects the heme to the protein?
His F8; so when oxygen binds to the heme group, the iron sinks further down into the ring and pulls on the attached helix indicating it had been oxygenated and a change in shape of the protein occurs affecting the other subunits without oxygen increasing their affinity for oxygen, increasing cooperatively
how does negative cooperatively affect Hb structure?
it makes it harder for O2 to bind
since we noted the sigmoidal cooperative oxygen binding curve what must this make Hb do?
Hb must be able to bind oxygen in the lungs, release oxygen in the capillaries; if Hb behaved like Mb, very little oxygen would be released
what type of heme protein is myoglobin?
monomeric heme protein
how does myoglobin bind with oxygen?
it cradles the heme group, the Fe in myoglobin Fe2+ is FERROUS IRON which is the form that binds oxygen which binds as the SIXTH LIGAND to Fe
what is the iron form found in myoglobin?
ferrous iron
oxygen binds as what numbered ligand to Fe in myoglobin?
sixth ligand
conformational change does not affect myoglobin, T/F?
F, conformational change does affect this monomer but is not communicated to any other myoglobin units
myoglobin and hemoglobin bind oxygen differently, T/F?
F, they bind similarly
describe how myoglobin and hemoglobin bind oxygen?
Oxygen binding changes the Myoglobin conformation
Without oxygen bound, Fe is out of heme plane
Oxygen binding pulls the Fe into the heme plane
Fe pulls its His F8 ligand along with it
The F helix moves when oxygen binds
This change means little to Myoglobin, but lots to Hemoglobin
what are the factors that modify hemoglobin O2 binding affinity?
1) blood pH
2) CO2 levels
3) 2,3 bisphosphoglycerate
4) Hb chain composition
the first threes are allosteric modulators that bind to hemoglobin and alter structure slightly, affecting cooperative binding that cause a shift in the O2 satuartion curve. The fourth point is a protein structure change at the amino acid level which changes Hb sensitivity to allosteric modulator 2,3- BPG
what is the significance of proteins having a net negative charge (acidic side chains)?
it allows them to pick up protons and serve as buffers and we see this with hemoglobin
what is the effect when you have the binding of protons to hemoglobin?
it causes structural changes that diminish oxygen binding
what is the effect when you have the binding of oxygen to hemoglobin?
it causes structural changes that diminish proton binding
what happens when protons are more plentiful? where do we see this
they bind to hemoglobin and reduce its affinity for oxygen; this is good for tissues:metabolism produces protons (lower pH), and metabolizing tissue needs more oxygen.
what happens when we have lower hemoglobin oxygen affintiy?
results in more oxygen for the tissues
what happens when protons are not abundant (higher pH)?
hemoglobin will lose protons and hemoglobin affinity for oxygen will increase. This is true in the lungs, where exhalation of CO2 results in higher pH. Under this condition, hemoglobin affinity for oxygen is much higher, allowing hemoglobin to pick up a lot of oxygen to carry to the tissues.
myoglobin is not a cooperative oxygen binding protein, T/F?
T
what is the effect of carbon dioxide on oxygen binding?
it diminishes oxygen binding
what is the effect of CO2 in tissues and extremities on the blood?
it leads to proton production in the blood taken up by hemoglobin as the oxygen dissociates
What is the effect when CO2 is low?
hydrogen ion concentration is low and so O2 affinity is increased
what is 2,3-BPG?
a molecule that affects hemoglobin oxygen affinity
what is the shape of the oxygen saturation curve in the absence of 2,3-BPG? What about in the presence of?
the oxygen binding to Hb follows a rectangular hyperbola; A sigmoid binding curve
2,3-BPG binds at a site distant from the Fe where oxygen binds, what effect is this?
2,3-BPG is an allosteric effector or allosteric modulator
when does 2,3-BPG production increase?
when oxygen levels decrease. So oxygen is released from hemoglobin when more oxygen is needed in the tissues
what is the cousin of 2,3-BPG?
1,3-BPG used in glycolysis
How does 2,3-BPG work?
it binds in a central cavity created by all four subunits on the hemoglobin protein.
describe the charges associated with 2,3-BPG
2,3-BPG negative charges interact with 2 Lys, 4 His, 2 N-terminal on hemoglobin; this effect results in a higher affinity for deoxy vs oxygen hemoglobin
what type of binding does 2,3-BPG have?
non-covalent and the binding depends on the affinity of hemoglobin for 2,3-BPG which depends on the structure of the pocket. Deoxy>oxy hemoglobin
what is sickle cell anemia the result of?
a single amino acid change where Glutamate-6 is changed to Valine 6 on the surface of the Hb Beta chain (HB S), this changes the shape of Hb molecule and becomes Deoxy Hb
So in sickle cell anemia, we know the Hb becomes deoxygenated, what results?
Deoxy Hb (S) will stick together and form large aggregates (clumps) (being a poor O2 transporter) with a rope-like fiber structure and polymerized Hb (S) fall out of solution inside red blood cells, cells take on a sickle shape clogging the capillaries and blocking blood flow in tissues
what is Hemoglobin A1c?
Because RBC’s have 120 day lifespan they can react with glucose and become glycosylated (non-enzymatic) and used to be detected in a blood test and generally diabetics will have more glycosylated Hb than patients with normal blood glucose.
how is HbA1c a convenient diagnostic test for patients?
its used to determine patients at risk for diabetes and for diabetics who are managing their blood glucose
