Proteins: Myoglobin And Hemoglobin and cytoskeleton 2 Flashcards
1
Q
Globular Proteins
A
- most numerous proteins
- distinguishable from fibrous proteins by the fact that they are soluble in aqueous media.
- core consists of fixed 2’ structures.
- Hydrophobic AA’s are oriented towards interior, Hydrophilic AA’s face the exterior.
- Overall structure is stabilized by H bonds, ionic interactions and less often S-S bonds.
- Because of the high degree of H bonding in the peptide backbone N-H and C=O are neutralized.
2
Q
Myoglobin and Hemoglobin
A
- most studied and best-understood globular proteins
- crucial in conversion of anaerobic life to aerobic life
- aerobic metabolism yields more energy than anaerobic
- due to O limited solubility in water, myoglobin and hemoglobin have evolved to deliver O to tissues in sufficient quantities.
3
Q
Myoglobin
A
- Single polypeptide chain of 153 AA’s
- contains a covalent lay bound heme group
- is the oxygen storage protein of muscle.
4
Q
Hemoglobin
A
- a2b2 tetramer, a subunit is 141 AA’s while b is 146 AA’s
- each polypeptide is structurally similar to myoglobin and each contains a heme
5
Q
Heme prosthetic group
A
O does not bind well to any AA, but it binds to various metals, including Fe and Cu
- Fe is frequently part of a complex with protoporhyrin IX
- porphyrin is composed of 4 pyrrole rings liked by methane bridges
- porphyrin is flat with two open sites for binding to Fe
- porphyrin is a prosthetic group, which helps proteins employ their functions.
- Protoporphyrin IX containing a bound Fe is called a heme
6
Q
Heme group binding
A
- Fe atoms of hemes typically exist as either +2 or +3
- O will only bind to +2 state when O binds the free heme it oxidizes it by one electron resulting in superoxide and ferric heme (+3)
- O bound heme is red, while heme with no O is purple
7
Q
Methemoglobin
A
Hemoglobin that has been oxidized from ferrous to ferric state. Thus unable to bind O.
8
Q
Effect of Protein Structure on O binding
A
- binding is influenced by structure
- cavity where O binds is created by Val (helix E) and Phe (located in the loop between helices C and D)
- There is no clear pathway for O binding to heme. Molecular motions (breathing) of the protein create transient routes to allow O to bind or leave. Rotation of the distal histidine is one important molecular motion
9
Q
Role of the Protein Scaffold
A
- heme cofactors is buried deep in the binding pocket composed primarily of alpha helix 2’ structure.
- the heme is held by numerous binding interactions as well as covalent interaction between the Fe and N atom of the histidine side chain in one of the helices
- coordination of the histidine to the heme completely blocks access of O to this face of the heme, forcing binding to the opposite face. This prevents oxidation of the heme by O and allows for simple reversible binding
10
Q
O binding to Heme
A
- O binding alters myoglobin conformation
- binding of O pulls the heme closer into the porphyrin ring. In turn, the histidine is pulled along, distorting the shape of the alpha helix.
- in hemoglobin O binding has profound effects on the ability of other subunits to bind O.
11
Q
T and R state of Hemoglobin
A
- two major conformations of hemoglobin as predicted by the models for allosteric activation
- O will bind to hemoglobin in either state, has higher affinity for R state
- In the absence of O hemoglobin is more stable in the T state, and is therefore the predominant form of deoxyhemoglobin. R stands for relaxed, T for tense which is stabilized by the greater number of ion pairs.
- transition of T to R causes rotation of alpha2beta2 pairs of subunits 15 degrees relative to the pair of alpha1beta1 subunits.
12
Q
Hill Plots for O2 binding to myoglobin and hemoglobin
A
- if Hill coefficient is 1 there is no cooperativity
- the maximum Hill coefficient is 3 which is less than the number of O binding sites in hemoglobin, which is normal for a protein that exhibits allosteric behavior
13
Q
Allosteric Effects in hemoglobin
A
- structure changes to oxygenation
- Increase in H+ causes Hb to release O2 (Bohr effect)
- 2,3-BPG binds to positively charged groups stabilizing deoxy Hb
14
Q
Carbonic anhydrase
A
- catalyzes rapid interconversion of CO2 and water to bicarbonate and H+ (or vice versa)
- Maintains the acid-base balance in blood and other tissues by transporting CO2 out of the tissues
15
Q
Bohr effect
A
- more O2 is released in tissues with higher absolute and/or relative CO2 values.
16
Q
2,3 - bisphosphoglycerate
A
- dramatically affects Hb’s binding of O2
- stabilizes deoxy Hb
- added during the storage of blood
- increased at high altitudes
17
Q
CO poisoning
A
- CO will bind to heme 20,000 times better than O, 200 times better when heme is bound to myoglobin
- it binds the heme in a bent conformation and a histidine residue makes a favorable H bond with it
- Preferred electronic configuration of CO binding to heme is in a linear conformation. The His residue at E7 steric ally hinders its binding to the heme, reducing its affinity
18
Q
Binding of NO in hemoglobin
A
- NO is a smooth muscle relaxing (hypotensive) agent
- at the tissues, some Hb binds NO instead of one of the 4 O
- NO is then transferred to Cys of beta subunit
- Upon releasing O, Hb passes NO to GSH (as GSNO), stabilizing NO
- NO transferred to receptors in vascular smooth muscle cells, relaxing them, facilitating O passage into tissues
19
Q
Subunit Composition of Hemoglobin Tetramers
A
The P50 values for HbA and HbF are 26 and 20 mm Hg
- in the placenta, this difference enables HbF to extract O from the HbA in the mother’s blood
- HbF is suboptimal postpartum since its high affinity for O2 limit the quantity of O2 delivered to the tissues
20
Q
Sickle cell anemia: HbS
A
- single nucleotide change in beta-glob in gene, Glu—>Val
- only if both genes are affected do you have sickle cell anemia
- normal rbc 120 days; HbS rbc 20 days
- sickling in low blood oxygen, or other conditions favoring deoxy Hb: low pH, high pCO2, high BPG
- fatigue and shortness of breath
- painful crisis
- Possible treatment: Silencing BCL11A normally suppresses the production of fetal hemoglobin
