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Hemoglobin, Collagen, and Elastin – Vocabulary Flashcards

Heme Proteins and Globular vs Fibrous Proteins: Hemoglobin and Collagen

  • Heme Proteins (Hemeproteins)

    • Contain a heme prosthetic group.
    • Examples: hemoglobin and myoglobin.
    • Belong to metalloproteins due to the presence of an iron ion bound at the center of the heme group.
    • Heme proteins are strongly colored, usually reddish-brown, due to the heme moiety.
    • The heme group is responsible for oxygen binding.

  • Globular vs Fibrous Proteins

    • Globular proteins include hemeproteins (e.g., hemoglobin, myoglobin).
    • Fibrous proteins include collagen and elastin (structural components of ECM).

  • Heme Structure (Fe-Protoporphyrin IX)

    • Central iron atom: Fe^{2+} (can form 6 bonds).
    • Flat, planar porphyrin ring composed of 4 pyrrole rings.
    • 4 nitrogen atoms coordinate the iron (Fe—N bonds to the pyrrole nitrogens).
    • Methene bridges link the pyrrole rings.
    • Tetrapyrrole ring features: 4 methyl groups, 2 vinyl groups, and 2 propionate side chains.
    • In the protein, Fe^{2+} binds directly to O_2 in the ferrous state.
    • In hemoglobin, Fe^{2+} also binds to a proximal histidine residue side chain on the globin chain (histidine from the globin molecule).

  • Heme Proteins: Functional Notes

    • Heme iron in Hb/myoglobin binds O_2; coordination environment can influence binding affinity.
    • Heme groups are arranged so iron–iron distances between subunits prevent direct heme–heme interactions (Hb tetramer distances ~24–40 Å) to avoid inter-heme interference.

Myoglobin vs. Hemoglobin

  • Myoglobin (Mb)
    • Monomeric protein.
    • Location: muscle tissue.
    • Function: intracellular oxygen storage.
  • Hemoglobin (Hb)
    • Tetrameric protein composed of two α-chains and two β-chains.
    • Structure forms heterodimers: α-chains and β-chains.
    • Hydrophobic interactions stabilize the αβ dimers.
    • Two identical αβ dimers (α1β1 and α2β2) form the Hb tetramer.
    • Each chain contains a heme Fe^{2+} that binds an oxygen molecule.
    • Hb can bind up to four O_2 molecules per tetramer.
    • The heme groups are well separated within the tetramer to minimize heme–heme interactions.
    • Overall transport role: Hb carries O2 from lungs to tissues and also participates in transport of H^+ and CO2 from tissues to lungs.

Oxygen Transport and Allosteric Regulation

  • Hb function vs Mb function

    • Hb O_2 binding is regulated by allosteric effectors; Mb binding is not.

  • Oxygen Transport in the bloodstream

    • Hb can carry up to 4 O_2 molecules from lungs to tissues.
    • In blood, Hb also transports protons (H^+) and CO_2 from tissues to the lungs as part of systemic gas exchange.

  • Deoxyhemoglobin (T state) and Oxyhemoglobin (R state)

    • Deoxyhemoglobin (T, taut) state: O2 is absent from iron; iron is pulled out of the plane of the heme; increased hydrogen bonding between αβ dimers; Hb has lower O2 affinity.
    • Oxyhemoglobin (R, relaxed) state: O2 binds to Fe; Fe moves into the plane of the heme; proximal histidine moves with Fe; altered αβ dimer interface promotes additional structural changes; Hb has higher O2 affinity.

  • Structural transition during O_2 binding

    • Upon O_2 binding, the heme Fe moves into the plane of the porphyrin; proximal histidine shifts along with Fe; movement of the histidine-containing α-helix alters the αβ dimer interface, triggering further conformational changes that propagate to other subunits.

  • Oxygen Dissociation Curve (ODC)

    • Myoglobin: hyperbolic curve; binds O2 strongly at low pO2 (in muscle).
    • Hemoglobin: sigmoidal curve due to cooperativity among subunits.
    • Po2 at half-saturation (P50): Myoglobin ≈ 1 mmHg; Hemoglobin ≈ 26 mmHg.
    • Tissue pO2 is much lower than in the lungs, favoring O2 offloading to tissues.
    • Note: The curve shape reflects the allosteric regulation and cooperative binding among Hb subunits.

  • Cooperativity and Oxygen Delivery

    • Cooperativity: O_2 binding to one Hb subunit increases affinity at remaining subunits, enhancing loading in lungs and unloading in tissues.
    • Allosteric effectors modulate HbO_2 affinity and the transition between T and R states.

Allosteric Regulation of Hemoglobin

  • Allosteric Effectors (general concept)

    • An effector ligand binds to a site on the enzyme (or protein) and alters the properties of another site that binds ligands.
    • Homotropic allosteric effect: same ligand acts at multiple sites (e.g., O_2 for Hb).
    • Heterotropic allosteric effect: different ligands influence binding (e.g., O_2 and 2,3-BPG).

  • Key allosteric effectors for Hb

    • pO2 (partial pressure of O2)
    • pH (Bohr effect)
    • pCO_2
    • 2,3-Bisphosphoglycerate (2,3-BPG)

  • 2,3-BPG as an allosteric effector

    • 2,3-BPG is produced in glycolysis and is highly anionic; in red blood cells, its concentration is ~2 mM, about the same as Hb concentration.
    • It binds in the central pocket of deoxyhemoglobin (T-state) via ionic interactions between negatively charged phosphates of 2,3-BPG and positively charged amino acids in the Hb central cavity.
    • Binding stabilizes the T (deoxy) form and reduces Hb's O_2 affinity.
    • During the transition from T to R, the central pocket is displaced, breaking Hb–2,3-BPG interactions and promoting oxygen release as O_2 affinity shifts higher.
    • Absence of 2,3-BPG yields high O2 affinity; presence shifts the oxygen dissociation curve to the right, favoring O2 release to tissues.
    • Physiological relevance: elevated 2,3-BPG in conditions like emphysema or anemia to promote greater O2 unloading; high 2,3-BPG helps compensate for reduced O2 delivery.

  • Bohr effect (pH and CO_2 effects)

    • Heterotropic regulation: increased H^+ (lower pH) and increased CO2 promote O2 release from Hb in tissues.
    • CO2 transport in blood occurs via three forms: carbaminohemoglobin (≈ 23%), bicarbonate (≈ 70%), and dissolved CO2 (≈ 7%).
    • Mechanism: in metabolically active tissues, carbonic anhydrase converts CO_2 to carbonic acid, which dissociates to bicarbonate and H^+.
    • ext{CO2} + ext{H}2 ext{O}
      ightleftharpoons ext{H}2 ext{CO}3
      ightleftharpoons ext{HCO}_3^- + ext{H}^+
    • Increased H^+ protonates histidine residues on deoxyHb, stabilizing the T-state and promoting O_2 release.
    • Relationship: lower pH or higher pCO2 stabilizes the T form and enhances O2 unloading; higher pH or lower pCO2 favors the R form and O2 loading.

  • Carbon Monoxide (CO) binding to Hb

    • CO binds to Hb heme sites with ~220x higher affinity than O_2.
    • Binding of CO shifts Hb to the R conformation and increases overall Hb O2 affinity, which reduces O2 release to tissues (leftward shift of the ODC).
    • CO binding also blocks O2 binding at other sites due to higher Hb affinity for CO than for O2.

Fetal Hemoglobin and Hemoglobin Variants

  • Hemoglobins exist as tetramers with two α-like and two β-like chains; globin genes are developmentally regulated.
  • Fetal Hemoglobin (HbF)
    • Composition: α2γ2 (two α chains and two γ chains).
    • HbF has higher O2 affinity than maternal HbA to facilitate transfer of O2 from maternal to fetal blood.
    • HbF does not bind 2,3-BPG as effectively as HbA, contributing to higher oxygen affinity.
    • As development proceeds, HbF is gradually replaced by HbA; HbF synthesis can be substantial in the fetus (about 60% of fetal Hb near birth).
    • Oxygen affinity of HbF decreases relative to late gestation as HbA becomes more prevalent, increasing 2,3-BPG sensitivity and reducing O_2 affinity.
  • HbA1c (Glycated Hemoglobin) and Diabetes Diagnostics
    • HbA1c reflects nonenzymatic glycation of hemoglobin A, used as a diagnostic/regulatory biomarker for diabetes.
    • Erythrocyte lifespan ~120 days; HbA1c reflects mean blood glucose over the past ~90 days.
    • Carbohydrate exposure leads to glycation of HbA, and HbA1c measurement provides guidance for diabetes management.

Hemoglobinopathies and Other Hb-Related Disorders

  • Sickle Cell Disease (HbS)
    • Mutation: Glu^6 → Val^6 in the β-globin chain.
    • HbS polymerizes when deoxygenated, causing red blood cells to assume a sickled shape.
    • Sickled RBCs can block small blood vessels, causing tissue hypoxia and pain.
    • Sickle cell trait provides some resistance to malaria.
    • HbS vs HbA differences contribute to polymerization under hypoxic conditions.
    • Summary points:
    • Point mutation: ext{Glu}^{6}
      ightarrow ext{Val}^{6} in β-globin.
    • Deoxygenated HbS polymerizes into fibers that distort RBCs.
    • Rigid RBCs impede blood flow and cause hypoxic pain episodes.
  • Methemoglobinemia
    • Methemoglobin contains Fe^{3+} (ferric) instead of Fe^{2+} (ferrous) in the heme iron.
    • Methemoglobin levels >1% cause altered blood color; >15% leads to neurologic and cardiac symptoms due to hypoxia; >70% can be fatal.
    • Causes:
    • Acquired: exposure to drugs or oxidants.
    • Congenital: defect in NADH-cytochrome b5 reductase.
  • Thalassemias
    • Hereditary blood disorders characterized by an imbalance in synthesis of hemoglobin chains.
    • Lead to anemia and fatigue; most common single-gene disorder in humans.
    • Partial or total absence of α or β globin chains reduces Hb production.
    • Similar to sickle cell, mutations may confer some malaria resistance.
  • α- and β-thalassemias specifics
    • α-thalassemias: deletions at the α-globin locus; four alleles encoding α-globin.
    • Lose 1 copy: silent carrier; lose 2 copies: α-thalassemia trait; lose 3 copies: hemolytic anemia with variable severity; lose 4: lethal (fetal).
    • β-thalassemias: point mutations leading to non-functional mRNA.
    • β-thalassemia minor: lose one β-globin gene; asymptomatic.
    • β-thalassemia major: lose both β-globin gene copies; severe anemia.
    • Treatments: blood transfusions are common.

Fibrous Proteins: Collagen and Elastin

  • Collagen: structural fibrous protein of the extracellular matrix (ECM)

    • Most abundant protein in the ECM and in the body overall (≈ 25–30% of total body protein).
    • Found in connective tissues (tendons, ligaments) and provides strength.
    • Structure: rigid triple helix formed by three intertwined polypeptide chains; contains post-translationally modified amino acids (hydroxyproline and hydroxylysine).
    • Function: cross-linking between helices via hydrogen bonds helps stabilize the triple helix.

  • Collagen Structure Details

    • The three polypeptide α-chains form individual right-handed helices.
    • Organization: α-chain → collagen molecules → collagen fibrils → collagen fibers.
    • Many collagen subtypes; all are triple helices but can be dispersed in ECM or tightly packed in parallel fibers (e.g., tendons).
    • Glycine residue at every third position: Gly-Xaa-Yaa (often Xaa = Proline, Yaa = Hydroxyproline).

  • Collagen Types (examples and roles)

    • Type I, II, III: fibril-forming collagens, linear fibrillar structures with high tensile strength; types I, II, III are major structural collagens in tissues.
    • Type IV and VIII: network-forming collagens that form meshworks or sheets in basement membranes.
    • Types IX and XII: fibril-associated collagens with interruptions that allow interaction with other ECM components.

  • Collagen Synthesis

    • Cells: fibroblasts, osteoblasts, chondrocytes produce collagen.
    • Prepro-α chains contain signal peptides; delivered to rough ER.
    • In the ER, signal peptides are cleaved to form pro-α chains.
    • Post-translational modifications
    • Prolyl hydroxylase hydroxylates proline and lysine to hydroxyproline and hydroxylysine to maximize H-bonding in the triple helix.
    • Glycosylation occurs on some hydroxylysine residues.
    • Three Pro-α chains assemble into procollagen triple helix with disulfide bonds.
    • Procollagen is transported to the Golgi, packaged into vesicles, and secreted into the ECM.
    • N- and C-terminal propeptides are cleaved by procollagen peptidases to form tropocollagen.
    • Tropocollagen spontaneously associates into collagen fibrils (overlaps of ~3/4 between adjacent fibrils).
    • Lysyl oxidase catalyzes cross-links between lysine/hydroxylysine residues on neighboring collagen molecules (forming allysine and hydroxyallysine) to yield mature collagen fibers.

  • Collagen Degradation and Remodeling

    • Collagen has a long half-life (often years).
    • Connective tissue remodeling occurs in response to growth and injury.
    • Collagenases (metalloproteinases) remodel collagen by cleaving fibrils.

  • Collagen-Related Diseases

    • Scurvy: Vitamin C (ascorbic acid) deficiency impairs prolyl and lysyl hydroxylation (requires Fe^{2+} and O_2), destabilizing the triple helix and interchain H-bonds; leads to capillary fragility, gum disease, poor wound healing.
    • Ehlers-Danlos Syndrome (EDS): heterogeneous group of connective tissue disorders; classic type involves type V collagen; features include skin extensibility and joint hypermobility; vascular type involves vascular problems and can be lethal due to arterial rupture; often caused by mutations in collagen-processing enzymes (e.g., lysyl hydroxylase) or collagen subunits.
    • Osteogenesis Imperfecta (OI): brittle bone syndrome; usually due to dominant mutations in α1 or α2 chains of type I collagen; glycine replacement with bulky amino acids disrupts triple-helix formation; ranges from mild to lethal (Type I vs Type II).
    • Dentinogenesis Imperfecta: tooth development disorder linked to OI; discolored teeth with wear susceptibility; treatment can involve bisphosphonates.
    • Alport syndrome: inherited disease with mutations in type IV collagen gene; basement membrane defects affecting kidney, cochlea, and eye; treatment includes dialysis or kidney transplant.

  • Elastin

    • Elastin is a fibrous protein providing elastic properties in connective tissue (e.g., lungs, large arterial walls, elastic ligaments).
    • Composed of elastin with glycoprotein microfibrils; forms insoluble polymers with tropoelastin as the precursor.
    • Tropoelastin (~700 amino acids) is secreted into the ECM, rich in small nonpolar amino acids (glycine, alanine, valine) and abundant in proline and lysine.
    • Fibrillin serves as a scaffold for tropoelastin deposition and interacts with glycoprotein microfibrils.
    • Lysyl oxidase catalyzes cross-linking by oxidizing lysine residues on tropoelastin to form allysine; condensation reactions between allysine and lysine residues form desmosine cross-links.
    • Mutations in fibrillin-1 cause Marfan syndrome (tall stature, long limbs, flexible joints; potential heart and vascular defects).

  • Alpha-1 Antitrypsin (AAT) and Elastin Diseases

    • AAT is a protease inhibitor that protects elastin in the lungs from neutrophil elastase.
    • Mutations in the AAT gene can produce nonfunctional misfolded protein, allowing elastase to degrade elastin in the alveolar walls, contributing to emphysema.
    • Smoking can oxidize methionine 358 in AAT, reducing its ability to bind proteolytic enzymes, causing permanent alveolar damage.
    • Therapeutic approach includes weekly intravenous AAT replacement therapy.

Summary of Key Concepts and Connections

  • Heme Proteins and O_2 Binding

    • Heme iron in Hb and Mb binds O2; Hb’s binding is allosterically regulated by pO2, pH, pCO_2, and 2,3-BPG.
    • Hb transitions between T (deoxy) and R (oxy) states drive cooperative O_2 binding and release.
    • The central pocket of Hb binds 2,3-BPG in the T-state to reduce O2 affinity, enabling O2 delivery to tissues when needed.
    • CO binding increases Hb's O2 affinity and prevents O2 release, leading to tissue hypoxia.

  • Fetal vs Adult Hb and Diabetes Diagnostics

    • HbF has higher O2 affinity to transfer O2 from mother to fetus; reduced 2,3-BPG sensitivity shifts HbF’s oxygen binding dynamics.
    • HbA1c serves as a diagnostic biomarker for long-term glucose exposure; reflects mean blood glucose over roughly 90 days due to erythrocyte lifespan (~120 days).

  • Structural and Functional Themes in ECM Proteins

    • Collagen provides structural integrity in the ECM through a stable triple helix and cross-links; misregulation or genetic mutations lead to connective tissue diseases.
    • Elastin provides elasticity; cross-linking and fibrillin scaffolding enable stretch and recoil, with Marfan syndrome arising from fibrillin-1 mutations.
    • ECM homeostasis involves synthesis, secretion, cross-linking, and proteolytic remodeling by metalloproteinases (collagenases).

  • Disease Links to Structure and Regulation

    • Sickle cell disease illustrates how a single amino acid substitution (Glu^6 → Val^6 in β-globin) can drive polymerization and altered RBC mechanics, contributing to vaso-occlusion and hypoxia.
    • Methemoglobinemia shows how iron oxidation state (Fe^{2+} vs Fe^{3+}) affects oxygen transport and tissue oxygenation.
    • Thalassemias demonstrate how imbalances in globin chain synthesis disrupt Hb production and oxygen transport.
    • Vitamin C deficiency (scurvy) and defects in collagen-processing enzymes underlie destabilized collagen and compromised connective tissue.

  • Formulas and Key Reactions (LaTeX)

    • Hemin oxygen binding context and structural transitions are described qualitatively; explicit quantitative expressions include:
    • Heme environment: ext{Fe^{2+}} ext{ binds O_2; proximal histidine interacts with Fe}
    • Fe–Fe distances in Hb tetramer: d_{ ext{Fe-Fe}} \approx 24\text{ Å} o 40\text{ Å}
    • P50 values: P{50, ext{Mb}} = 1\ \text{mmHg},\quad P_{50, ext{Hb}} = 26\ \text{mmHg}
    • Bohr reaction: \text{CO2} + \text{H2O} \rightleftharpoons \text{H2CO3} \rightleftrightarrow \text{HCO_3^-} + \text{H^+}
    • HbF composition: \text{HbF} = \alpha2\gamma2
    • HbA1c concept: HbA1c reflects average glucose exposure over ~90 days due to RBC lifespan ~120 days.

  • Connections to Foundational Principles

    • Ligand binding cooperativity and allostery illustrate emergent properties of multi-subunit proteins beyond simple one-site binding.
    • Protein structure–function relationships: specific residues (e.g., distal/proximal histidines, glycine in collagen) are critical to function.
    • Post-translational modifications (hydroxylation in collagen, cross-linking in elastin) are essential for mature protein function.
    • Evolutionary aspects: Hb variants and HbF regulation reflect adaptation to different physiological needs (fetal development, malaria resistance).

  • Practical/Clinical Relevance

    • Understanding Hb allostery helps explain oxygen delivery in health and disease (e.g., high-altitude adaptation, anemia conditions).
    • HbA1c testing is a standard diabetes management tool; interpretation requires knowledge of RBC lifespan and glycation kinetics.
    • Sickle cell disease, thalassemias, methemoglobinemia, scurvy, and connective tissue disorders illustrate how protein structure and modification state translate to clinical phenotypes and treatment strategies.

  • Ethical/Societal Considerations (brief)

    • Genetic disorders (sickle cell, thalassemias, Alport, EDS) raise considerations for screening, counseling, and access to therapy.
    • Management of chronic Hb-related conditions (AAT deficiency, diabetes) intersects with public health and preventive care.

  • Quick Reference Highlights

    • Hb tetramer: two α and two β chains; four heme groups; cooperative O_2 binding with allosteric modulation.
    • Central Hb pocket binds 2,3-BPG in the T-state; reduces O2 affinity; release in tissues is promoted by Bohr effect (low pH, high pCO2).
    • Fetal Hb (HbF) has higher O2 affinity and reduced 2,3-BPG binding compared to HbA, facilitating fetal O2 uptake.
    • CO binding to Hb causes a left shift and inhibits O_2 release to tissues.

  • Note on Nomenclature

    • HbA: adult hemoglobin (α2β2).
    • HbF: fetal hemoglobin (α2γ2).
    • HbS: sickle cell hemoglobin with Glu^6 → Val^6 mutation in β-globin.
    • HbA1c: glycated hemoglobin used in diabetes management.

  • See-Also (related lipids and membranes topic upcoming)

    • Lipids and membranes topics build on protein–lipid interactions and membrane protein function, complementing the understanding of ECM and cytoskeletal components in tissue structure.