enzymes

• Ligand binding


  Reversible binding of a ligand to a protein
• Myoglobin


  Oxygen storage protein in skeletal muscle
• Hemoglobin


  Oxygen transport protein in blood
• Catalysis


  The acceleration of a chemical reaction by an enzyme
• Enzyme properties


  • Enzymes
  • Enzyme kinetics
  • Enzyme mechanism
• Topic 5: Protein function
• Myoglobin


  • Function
  • Oxygen binding curve
  • Structure
• Hemoglobin


  • Structure
  • Function
  • Oxygen binding curve
  • Factors influencing oxygen binding
  • High altitude adaptation
  • Different globins
• Enzymes


  • Definition
  • How they achieve catalysis
  • Cofactors
  • Example of enzymatic reaction: lysozyme mechanism
  • Michaelis-Menten model
  • Factors influencing enzyme activity
• Reversible ligand binding


  Binding of a ligand to a protein that can be reversed
• Reversible ligand binding is crucial in biological systems
• Fractional saturation


  A measure of ligand binding, the ratio of bound ligand to total binding sites
• Hemoglobin and Myoglobin


  • Hb function: O2 transport from lungs to tissues
  • Mb function: O2 storage in skeletal muscle
• Heme prosthetic group enables precise control of O2 binding
• Myoglobin


  • 153 amino acids
  • All alpha helices
  • Globin fold with 8 helices A-H
  • Heme as prosthetic group
• Oxygen binding to Mb

  Hyperbolic curve, fractional saturation (YO2) vs partial pressure of O2 (PO2)
• Oxygen binding to Mb can be measured by visible spectroscopy

  
• Oxygen binding site in Mb

  • His F8 and His E7 residues control access of O2 to Fe(II) in heme
• Protein environment controls ligand binding affinity in Mb
• Hemoglobin


  • Consists of 4 globin subunits: 2 alpha, 2 beta
  • Heterotetramer structure: alpha2beta2
• Hemoglobin function and O2 binding

  • In lungs: binds O2 at high pO2
  • In tissues: releases O2 at low pO2
• Hemoglobin O2 binding curve


  Sigmoidal shape, with different saturation levels in lungs vs tissues
• Hemoglobin


  • Two states: T (tense) and R (relaxed)
  • T state has lower O2 affinity, R state has higher O2 affinity
• Allosteric protein


  Binding of a small molecule to one site causes conformational change that alters binding at another site
• Allosteric effectors of Hemoglobin

  • O2 is a homoallosteric activator
  • BPG, H+, CO2 are allosteric inhibitors
• O2 binding to Hb leads to conformational changes, including subtle movement of helix F
• BPG


  2,3-bisphosphoglycerate, binds selectively to deoxyHb (T state)
• Effect of BPG on O2 binding

  Increases O2 release at tissues
• Effect of altitude on BPG and O2 binding

  Increased BPG at high altitude shifts O2 binding curve to right, increasing O2 release
• Bohr effect


  Decrease in pH (increase in H+) favors T state of Hb, decreasing O2 affinity
• Physiological significance of Bohr effect is to facilitate O2 release in respiring tissues
• Effects of CO2


  • CO2 can bind directly to Hb, forming carbamate
  • Increases stabilization of T state
• Proportion of Hb in T vs R state depends on pO2, [BPG], [H+], [CO2]
• Sickle-cell hemoglobin (HbS)


  Mutation E6 to V leads to polymerization of deoxyHbS, causing sickle-shaped red blood cells
• Sickle-cell anemia


  • Sickled cells clog blood vessels, causing inflammation, pain, anemia, organ damage, stroke
  • If heterozygous, provides some resistance to malaria
• Fetal hemoglobin (HbF)


  Alpha2gamma2 structure, higher O2 affinity than adult HbA
• Failure to switch from HbF to HbA is linked to Sudden Infant Death Syndrome
• HbF production can be artificially stimulated by hydroxyurea, used to treat hemoglobin disorders
• Enzymes


  Biological catalysts, work under mild conditions, highly specific
• Every reaction in the cell is catalyzed by an enzyme