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Hemoglobin & Myoglobin..
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04.26.06 (2 years ago)
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Both contain heme, a cyclic tetrapyrrole consisting of 4 molecules of pyrrole linked by Alpha-methylene brodges.
Mb is monomeric & Hb is tetramer of 2 subtypes. though different primary structures, Mb & subunits of Hb have nearly identical secondary & tertiary structures.
Histidines F8 & E7 perform unique roles in O2 binding.
Mb stores O2 & Hb transports O2. This is so because O2 binding curves for them are hyperbolic & sigmoid, respectively. so Mb releases only a small fraction of bound O2 at PO2 values encountered in active muscle(20 mm Hg) or other tissues(40 mm Hg).
Cooperative binding: a molecule of O2 binds to a Hb tetramer more readily if other O2 molecules are already bound.
P50: it is the partial pressure of O2 that half saturates a given Hb. values for HbA & HbF are 26 & 20 mm of Hg respectively.
By the end of 1st trimester, embryonic Hb is replaced by HbF. Synthesis of beta subunits begins in 3rd trimister.
Bohr effect: reciprocal coupling of proton & O2 binding to Hb.
2,3-BPG: low PO2 in peripheral tissues promotes its synthesis from 1,3-BPG, a glycolytic intermediate. Hb tetramer binds one molecule of it in a central cavity formed by its 4 subunits. this space is wide enough to accommodate BPG only when Hb is in T state.
Residue H21 of gamma subunit of HbF is Ser rather than His which cannot form a salt bridge. so BPG binds more weakly to HbF than to HbA, hence higher affinity for O2.
High altitude elevates BPG that lowers affinity of HbA for O2.
Methemoglobinemia: heme iron is ferric. it may be d/t:
oxidation of Fe2+ to Fe3+ by sulphonamides
hereditary HbM
reduced activity of MetHb reductase
HbM: histidine F8 is replaced by Tyrosine. iron of HbM forms a tight ionic complex with the phenolate anion of tyrosine that stabilizes Fe3+ form
Hb Chesapeake: favours R state increase O2 affinity, so fail to deliver adequate O2 to peripheral tissues, so tissue hypoxia, so polycythemia.
HbS: nonpolar Valine replaces polar surface of Glu6 of beta subunit, generating hydrophobic 'sticky patch' on surface of both oxyHb & deoxyHb. both HbA & HbS contain complementary sticky patch on their surfaces that is exposed only in deoxygenated R state.so at low PO2 deoxyHbS can polymerize to form long insoluble fibres.
HbA1c: blood glucose glycosylates amino group of lysine residues & the amino terminals of Hb.
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