5Introduction to Heme Proteins Biomedical importanceStructure of hemeStructure and function of MbStructure and function of HbO2 saturation curve of Mb and Hb differencesThe Bohr effectEffect of 2, 3 BisphosphoglycerateHemoglobinopathiesMet HbHbS and thalassemias
6Biomedical Importance Heme proteins functionO2 bindingO2 transportElectron transportSickle cell disease: subunit of Hb altered.Cyanide and CO poisoning2, 3 BPG (Bisphosphoglycerate) effect on Hb.
8Heme Proteins Mb and Hb are “heme” proteins. Porphyrins are colored. Iron porphyrin gives red color to blood.Magnesium porphyrin gives green color to plants.Fe2+ + protoporphyrin IX = HEMEHeme is a prosthetic group, or non-polypeptide unit necessary for the biological activity.FerriMb-Iron is +3 oxidation stateOxyMb-Iron is +2 oxidation state and O2 is bound to 6thDeoxyMb-Iron is +2 oxidation state
9Heme Structure Four pyrolles linked methene bridges Four CH3, 2 vinyl, and 2 propionate side chains could be arranged in fifteen different ways.Only one isomer, protoporphyrin IX, present.Fe2+ Four nitrogen atomsFe2+ normally octahedrally coordinated.6 ligands4N of the porphyrin ring available2 remainingIn cytochrome c 5th and 6th met and HisIn Mb and Hb 5th coordinated to a His, 6th is for O2
10Structure and Function of Myoglobin Myoglobin, a heme protein in heart and skeletal muscles.Myboglobin functions asreservoir for O2.carrier for O2.Myoglobin is single polypeptide chain (MW 17,000), 153 amino acidsSurface is polar, interior is non-polar (characteristic pattern of globular proteins).75% of polypeptide chain folded into eight stretches of -helical regions labeled A-H. Four segments terminated by proline.
11Structure and Function Continued Heme group of Mb in crevice in proteins (between E and F helices). Cavity lined with non-polar amino acids, except two His residues.Proximal His binds Fe2+.Distal His (E7) does not directly interact with heme group but helps stabilize ferrous form of iron.Therefore, Mb creates special environment for heme that allows reversible binding of oxygen without simultaneous oxidation of Fe2+.His F8 means 8th residue in F helix, it identifies it as a histidyl residue.F8 proximal HisE7 distal His
12More Structure and Function CO binds 25,000 times stronger than O2 in isolated heme.Because no distal His in isolated heme. In biological systems, distal His forces CO to bind at an angle, decreasing affinity of CO to heme (200 times). This protects us against CO poisoning.Natural protection.
13Electron-density Map of Myoglobin Near the Oxygen-binding Site
27Benefit of Sigmoidal Curve Why Mb unsuitable as O2 transport proteinPO2 in lung 100 mmHgPO2 n venous blood 40 mmHgIt cannot serve as an effective vehicle for delivery of O2 from lungs to peripheryAfter exercise PO2 of muscle 5 mmHg. Then Mb releases its O2Sigmoidal curve allows Hb to deliver more O2 to tissues in response to small S in PO2
28Salt links between and within subunits in deoxyhemoglobin
49Hb has allosteric properties Allosterism (Allos: other, Steros: space): enzyme regulation that effector binds to one site on enzyme and increases or decreases the activity of another site.
50Sickle Cell Anemia Genetically transmitted disease 4/1000 among blacksSickle cell anemia homozygousSickle cell trait is heterozygousHbS: Sickle cell Hb; HbA: Normal HbThe only difference is substitution of Val for Glu at position six of the beta chains!Fetal DNA can be analyzed for sickle cell geneVal Glu results from T A mutationDetected by new techniquesSickle cell gene has missing cleavage site for restriction endonuclease which produces 1.3 kb fragment instead 1.1 kb fragment on gel electrophoresisDNA analysis performed early in pregnancy (8 weeks gestation)
55Why HbS aggregates? DeoxyHbS New hydrophobic patch Val6 interacts Phe85 and Val88 of beta chain of neighboring molecule to initiate aggregation.Why aggregates do not form when HbS oxygenatedIn OxyHbS Phe85 and Val88 are largely buried inside Hb….