3. Globular Proteins- Hemoglobin Flashcards
Types of proteins-
Simple
purely AAs
Types of proteins-
Complex
With a prosthetic group (e.g., lipid, sugar, other non-protein)
Types of proteins-
Globular
AA seq?
(2 pics, table vs fibrous)
Spherical
AA sequence: Quasi random of hydrophilic and hydrophobic AAs
Example: Hemoglobin
Disease: Sickle-cell anemia
–
Shape- rounded, spherical
Role- functional (catalytic, transport, etc)
Solubility- (generally) soluble in water
Sequence- irregular AA sequence
Stability- more sensitive to changes in heat, pH, etc vs fibrous protein
Ex- catalase, hemoglobin, insulin, immunoglobin
Types of proteins-
Fibrous
AA seq?
Long, rope-like
AA sequence: repeats
Example: Collagen
Disease: Osteogenesis Imperfecta (OI)
–
Shape- long, narrow
Role- structural (strength, support)
Solubility- (generally) insoluble in water
Sequence- repetitive AA sequence
Stability- less sensitive to changes in heat, pH, etc vs globular protein
Ex- collagen, myosin, fibrin, actin, keratin, elastin
Types of proteins-
Membrane
AA seq?
Transmembrane
AA sequence: blocks
Example: Insulin receptor
Disease: Diabetes mellitus
Globular oxygen binding proteins (classic ex)-
Myoglobin
Monomeric
153 aa, 17,200 kDa
Heme b prosthetic group with Fe2+
In cardiac and skeletal muscle
Globular oxygen binding proteins (classic ex)-
Hemoglobin
Tetrameric 2 α and 2 β chains α chain, 141 aa β chain, 146 aa Each chain has Heme b prosthetic group with Fe2+ In erythrocytes
Hemoglobin Function
(pic)
Hb must bind oxygen in lungs and release it in capillaries
Binding of first oxygen to heme of Hb draws Fe into the plane of the porphyrin ring
Leads to a series of conformational changes that are transmitted to adjacent subunits
Adjacent subunits’ affinity for oxygen increases
This is called positive cooperativity
Results in a sigmoid (S-shaped) binding curve
Both O2 and CO Bind to Fe2+ of Heme
Affinity of Hb for CO is 230 times that for O2
Oxygen Binding Curves for Mb and Hb
P50 – Partial pressure of O2 at which 50% saturation occurs
O2 binding affinity
- Mb»_space; Hb
- Mb – for oxygen storage
- Hb – for oxygen transport
Hyperbolic O2 binding curve for Mb
Sigmoidal O2 binding curve for Hb
oxygen-disassociation curve steepest at oxygen concentrations that occur in tissues (vs curve ends in lungs)
permits O2 delivery to respond to small changes in pO2
Clicker-What is advantage of myoglobin reaching higher values of % sat at lower oxy partial pressure than Hb. Advantage=serving as oxy reserve for muscle in event of hypoxia
T and R States of Hb
(pic)
The equilibrium between T and R is modulated by effector molecules that tell hemoglobin when to bind or release O2
T/taut structure of deoxyHb
- weak ionic and H bonds btwn alpha-beta dimer pairs
- strong interaxns, primarily hydrophobic, btwn alpha and b chains form stable alpha-beta dimers
R/relaxed structure of oxyHb
-some ionic and H bonds btwn alpha-beta dimers broken in oxygenated state
The Bohr Effect
Competition between oxygen and H+
Discovered by Christian Bohr
*Binding of protons diminishes oxygen binding
*Binding of oxygen diminishes proton binding
Important physiological significance
Decrease in pH results in decreased O2 affinity of Hb and thus, shift to right in O2 dissociation curve
At lower pH, greater pO2 req to achieve any given O2 saturation
Bohr Effect II
Carbon dioxide diminishes oxygen binding
Binding of CO2 to Hb (CarbaminoHb) in peripheral tissues leads to proton production
-CO2 binds to amino-terminal residue, releasing H+ and producing carbamino-terminal residue
Hydration of CO2 by carbonic anhydrase in tissues also leads to proton production
-CO2 + H2O -> (carbonic anhydrase) -> H2CO3 (carbonic acid) -> (bicarbonate) HCO3- + H+ (all rxns reversible)
These protons are taken up by Hb as oxygen dissociates (*Hb acts as a buffer – Haldane Effect)
The reverse occurs in lungs (CO2 released from Hb, O2 binds to Hb (oxyHb))
Fine Tuning Oxygen Delivery
Hb alone binds oxygen a little too tightly
*2,3-Bisphosphoglycerate (BPG) is the primary modulator of Hb’s oxygen affinity
BPG binds in the central positively charged cavity of deoxyHb by forming ionic bonds with the β subunits
BPG shifts the O2 binding curve to the right by stabilizing the T state
-eg 2,3-BPG=8 mmol/L (blood from indiv adapted to high altitudes)
BPG
Most abundant organic phosphate in blood
Made through intermediate metabolism:
*Glycolysis (glc -> 1,3-BPG -> 2,3-BPG -> 3-phosphoglycerate -> lactate)
[BPG] is approx equal to [Hb]
Role of BPG in transfused blood
Rapid depletion of BPG in stored red cells
Hb in BPG-depleted blood has abnormally high affinity for oxygen
-thus, poor delivery of oxygen to tissues after transfusion
Decrease in BPG prevented by supplementing stored blood with the nucleoside, inosine (hypoxanthine-ribose)
-Ribose from the inosine is converted to BPG in red cells
Fine Tuning Oxygen Binding:
Summary of Effectors
Oxygen binding to Hb (oxyHb) is promoted by O2
-*O2 promotes R state
Oxygen release from Hb (deoxyHb) is promoted by BPG, H+ and CO2
-*BPG, H+ and CO2 promote T state
Non-functional Hb
Carboxyhemoglobin (CO-Hb)
- *CO promotes R state and prevents the release of O2
- Increased levels of CO-Hb in heavy smokers and as a result of environmental pollution
- Can revert to oxyhemoglobin
Methemoglobin
- Iron in the ferric (Fe3+) rather than ferrous (Fe2+) state, incapable of binding oxygen
- *Result of strong oxidative drugs or a deficiency of methemoglobin reductase
- Can revert to oxyhemoglobin
Sulfhemoglobin
- Hemoglobin molecule contains sulfur
- Caused by certain sulfur-containing drugs or chronic constipation
- Cannot revert to oxyhemoglobin and may cause death
Globin gene families
2 Copies of α globin genes on Chromosome 16 (total = 4 genes)
1 Copy of β globin gene on Chromosome 11 (total = 2 genes)
δ (delta) globin is a minor adult version of β globin
γ (gamma) globin is the fetal version of β globin
ε (episilon) globin is the embryonic version of β globin
ζ (zeta) globin is the embryonic version of α globin
–
If want loss of gene, lose alpha
Absence of one beta gene causes more serious thalassemia than loss of alpha bc 4 alpha genes, 2 beta genes
Composition of Adult Hb
HbA- α2β2- 90% of total Hb
HbF- α2γ2- <2% of total Hb
HbA2- α2δ2- 2-3% of total Hb
HbA1c- α2β2-glucose- 4-6% of total Hb
HbF binds BPG weakly because γ chains interact poorly with BPG
Glucose is non-enzymatically added (glycation) to the amino terminal Val of β-globin chains in HbA1c
HbF vs HbA
More of HbF is in R State Than HbA at Low pO2
O2 flows from maternal oxyHb to fetal deoxyHb
Hemoglobinopathies – Qualitative
*Changes in amino acid sequence of Hb affect its properties
HbA: *no mutation
Normal adult hemoglobin
β-Globin chains have Glu at position 6
HbS
Sickle hemoglobin
β-Globin chains have Val at position 6
Tend to aggregate due to hydrophobic interactions
HbC
β-Globin chains have Lys at position 6
Less stable than HbA
Sickle Cell Disease
DeoxyHbS aggregates -Mutant β chains sticky Sickle Cells Obstruct Blood Flow -1. pt mutation in DNA codes for structurally altered HbS (Glu -> Val) -2. in deoxy state, HbS polymerizes into long, rope-like fibers Vascular occlusion Chronic anemia Periodic episodes of pain
Sickle Variations in Hb
AA = Normal Hemoglobin AS = Sickle Cell Trait SS = Sickle Cell Disease/Anemia AC = Hemoglobin C trait SC = Sickle Hemoglobin C Disease
Sickle cell disease is autosomal recessive
Most common among African Americans
- 1 in 12 has the trait
- 1 in 500 has the disease
Treatment includes blood transfusion and hydroxyurea (increases the expression of globin γ chains – HbF is not sticky)
Hemoglobinopathies – Quantitative
- Thalassemias - A group of autosomal recessive disorders characterized by deletion of one or more globin chains
- Unbalanced production of globin subunits
*α-Thalassemia – deletion of genes for α-chain
SE Asian, W Indian, Mediterranean, African descent
*β-Thalassemia – deletion of genes for β-chain
-Thalassemia major (βo) = (Cooley’s anemia): no β-chain production
-Thalassemia minor (β+) = rate of synthesis of β-chain reduced
Asian, Mediterranean, Middle East, Hispanic, W Indian
α-Thalassemia
Each copy of chromosome 16 has 2 genes for α-chain
Loss of one gene is asymptomatic
Loss of 2 genes causes mild anemia symptoms
Loss of 3 genes results in Hb H disease (clinically severe) due to aggregation of β tetramers
Loss of all 4 genes eg all gamma causes Hydrops fetalis (Hb Bart disease) – fetal death
β-Thalassemia
One gene for β-chain on each copy of chromosome 11
Loss of one causes β+
Loss of both results in βo
Anemia
