Hemoglobin & Myoglobin& Collagen

• Heme proteins• Supply of oxygen – Oxidative metabolism

• Myoglobin– Monomeric – protein of red muscle– Stores oxygen

• Hemoglobin– O2 transport– Tetrameric • Cooperative interactions

Normal Hemoglobins

• Hb A (α2β2)– Major normal adult hemoglobin• Comprising about 97% of the total

• Hb F (α2γ2)– Major hemoglobin of the fetus• Increased oxygen affinity• Lower affinity for 2,3-diphosphoglycerate (DPG)

• Hb A2 (α2δ2)– 1.5-3.5% of normal adult– Increased Hb A2

• β-thalassemias

• Embryonic Hemoglobins– Hb Gower-1 (ζ2ε2)

– HbPortland (ζ2γ2)

– Hb Gower-2 (α2ε2)

Developmental pattern of the quaternarystructure of fetal and newborn hemoglobins

• Heme & ferrous iron

• Fe2+ linked to all four nitrogen atoms of the heme, to histidine F8, and, in oxyMb & oxyHb, also to O2

Heme

Myoglobin

• O2 storage• Rich in α Helix• 153-aminoacyl residue• MW 17,000 • 75% in eight right-handed– Helices A–H

• Surface of myoglobin is polar• Interior contains only nonpolar– Leu, Val, Phe,

A model of myoglobin

Myoglobin

• Histidines F8 & E7 – Roles in Oxygen binding– Proximal histidine, His F8• The fifth coordination position of the iron

• O2 occupies the sixth coordination position

Hemoglobin

• Tetrameric – α2β2 (HbA)

– α2γ2 (HbF)

– α2S2 (HbS)

– α2δ2 (HbA2)

• the α polypeptide– Seven helical regions

• bind four molecules of O2 per tetramer

• Cooperative binding– A molecule of O2 binds to a hemoglobin tetramer

more readily if other O2 molecules are already bound

• P50 – expresses the relative affinities of different

hemoglobins for oxygen – The partial pressure of O2 that half-saturates Hb

• P50 for HbA and fetal HbF– 26 and 20 mm Hg – HbF,High affinity for O2

• On oxygenation of hemoglobin, the iron, histidine F8, and linked residues move toward the heme ring.

The iron atom moves into the plane ofthe heme on oxygenation. Histidine F8 and its associated residues are pulled along with the iron atom.

• Oxygenation of hemoglobin is accompanied by large conformational changes

• binding of the first O2

• Iron motion • rupture of salt bridges • T (taut) state to the R (relaxed) state– Low affinity and high-affinity conformations

The transition between the two structures is influenced by protons, carbon dioxide, chloride, and BPG; the higher their concentration, the more oxygen must be bound to trigger the transition.

• After releasing O2 at the tissues, hemoglobin transports CO2 & protons to the lungs

• CO2 as carbamates

• 15% of the CO2 in venous blood• Remaining

• One proton for every two O2 molecules released

• In the lungs, as O2 binds to deoxyhemoglobin, protons are released and combine with bicarbonate to form carbonic acid.

• Bohr effect– Reciprocal coupling of proton and O2 binding

• O2 Binds– Rupture of Salt Bonds• Proton release

The Bohr effect

• R state– Oxygenated – breaks salt bridges

• Release of O2

– the T structure • Salt bridges re-form

• Increase in proton concentration – Enhances the release of O2

• Increase in PO2 – Promotes proton release

• 2,3-Bisphosphoglycerate (BPG)– Stabilizes the T Structure of Hemoglobin• deoxyhemoglobin

• Low PO2

– Promotes the synthesis • Forming

additional salt bridge

Mode of binding of 2,3-bisphosphoglycerate to human deoxyhemoglobin. BPG interacts with three positively charged groups on each β chain.

• Adaptation to High Altitude– Increase in• Number of erythrocytes• Concentrations of hemoglobin• BPG

• Myoglobinuria– Massive crush injury• Urine dark red

– Myocardial infarction

• Anemias– Reductions in• Number of red blood cells

– Folic acid or vitamin B12 deficiency

• Hemoglobin – Iron deficiency

Glycosylated Hemoglobin (HbA1c)

• ε-amino group of lysine residues• Amino terminals• Normal,about 5%• Proportionate to blood glucose concentration• Reflects the mean blood glucose

concentration over the preceding 6–8 weeks• For management of diabetes mellitus

• Numerous mutant human hemoglobins have been identified– Hemoglobinopathies & Thalassemias

Collagen

• a Fibrous Protein • the most abundant of the fibrous proteins • 25% of the protein mass in body • Strength & flexibility of skin – Collagen & keratin fibers

• Bones & teeth• elongated proteins• repetitive amino acid sequences• regular secondary structure

Collagen

Primary, secondary, and tertiary structures of collagen

Collagen

• Rich in proline & hydroxyproline– repetitive Gly-X-Y pattern• Y generally is proline or hydroxyproline

Collagen maturation

• Collagen Is Synthesized as a Larger Precursor• Procollagen • Prolyl hydroxylase• Lysyl hydroxylase

Collagen

• Collagen triple helices are stabilized by– Hydrogen bonds• Residues in different polypeptide chains.• Hydroxyprolyl & hydroxylysyl

– Covalent cross-links between• Modified lysyl residues both within and between

polypeptide chains.

• Hydroxylysyl residues– Glucosyl or galactosyl residues

• Removal of the globular amino terminal & carboxyl terminal • Cross-link

– Certain lysyl residues are modified• Lysyl oxidase

– Copper-containing

• ε-amino groups to aldehydes– aldol condensation

» C=C double bond– C-N bond

– Strength & rigidity

Order and location of processing of the fibrillar collagen precursor

Disorders of collagen maturation

• Nutritional– Scurvy• Deficiency of vitamin C

– Prolyl & lysyl hydroxylases

• Bleeding gums, swelling joints, poor wound healing

– Copper Deficiency • Lysyl oxidase

Disorders of collagen maturation

• Genetic – Osteogenesis imperfecta

• Fragile bones

• Ehlers-Dahlos syndrome– Connective tissue disorders– Defects in the genes

• α collagen-1• Procollagen N-peptidase• Lysyl hydroxylase

– Mobile joints & skin abnormalities