Biochemistry I - Oxygen Binding by Myoglobin and Hemoglobin - Quick Guides | Biochemistry

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Study GuideBiochemistry I–Oxygen Binding by Myoglobin andHemoglobin1.Hemoglobin and MyoglobinHemoglobin and myoglobin are proteins that bind oxygen, but they are designed fordifferent rolesinthe body. Although theirprimary amino acid sequencesare only slightly related, the differencesbetween the two proteins are mostlyconservative substitutions.More importantly, theirsecondary structuresare almost identical. Both proteins are largelyα-helical, and the helices are arranged in a very similar way.1.1The Heme Group and Oxygen BindingBoth proteins bind oxygen using aheme group.•Each protein bindsone O₂molecule per heme•The oxygen binds to aniron atom (Fe²⁺)in the heme•The bond between oxygen and iron is acoordinate covalent bondHeme structure:•Square-planar molecule•Made offour pyrrole rings•Nitrogen atoms bindfour of iron’s six coordination sitesThe remaining two coordination positions:1.One binds to ahistidine side chainof the protein→ called theproximal histidine2.One bindsoxygen (O₂)

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Study GuideFigure 1: Heme group showing Fe, proximal histidine (F8), distal histidine (E7), and O₂bindingsite1.2Role of the Distal HistidineFree heme reacts poorly with oxygen because O₂can oxidizeFe²⁺to Fe³⁺, which binds oxygenweakly. This would make hemoglobin and myoglobin useless.This problem is prevented by thedistal histidine:•It forces oxygen to bind at abent angle•Prevents formation of a linear Fe–O–O bond•Reduces oxidation of iron•Results inweaker but reversible oxygen bindingWeaker binding is actually useful because it allowseasy oxygen release.1.3Structural Differences Between Hemoglobin and MyoglobinThe major functional difference between the two proteins lies in theirquaternary structure.Myoglobin•Monomeric(single polypeptide chain)•No quaternary structure

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Study Guide•Binds oxygenvery tightly•Found in muscle cells•Stores oxygen and facilitates its diffusionHemoglobin•Tetramer•Composed of:oTwoαsubunitsoTwoβsubunits•Binds oxygenless tightly•Found in red blood cells•Transports oxygen from lungs to tissuesThis difference in oxygen affinity allows oxygen to move from hemoglobin → myoglobin → musclecells.1.4Oxygen Binding by MyoglobinMyoglobin binds oxygen through asimple equilibrium reaction:Because myoglobin hasone heme, it can bind onlyone oxygen molecule.Oxygen Binding Curve of MyoglobinOxygen concentration is measured aspartial pressure (pO₂).The binding curve ishyperbolic, described by:

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Study GuideWhere:•Y= fraction of oxygen-bound myoglobin•pO₂= oxygen partial pressure (torr)•P₅₀= pO₂at which 50% of myoglobin is saturatedFigure 2: Hyperbolic oxygen-binding curve of myoglobin compared to hemoglobin1.5Physiological Meaning of P₅₀•Myoglobin P₅₀≈1 torr•Atmospheric pO₂≈159 torr•Therefore, myoglobin isalmost fully saturatedat normal oxygen levelsY ≈ 0.99 at pO₂= 159 torrIn blood:•Venous saturation ≈0.91•Arterial saturation ≈0.971.6Oxygen Binding by HemoglobinHemoglobin hasfour heme groups, so oxygen binding occurs infour steps:

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Study GuideThis shows thestepwise binding of oxygen to hemoglobin, up to four O₂molecules.Each oxygen-binding event affects the next one. This phenomenon is calledcooperativity, a type ofhomotropic effect.1.7Cooperative Binding and Sigmoidal CurveBecause of cooperativity:•Hemoglobin’s oxygen-binding curve issigmoidal (S-shaped)•Not hyperbolic like myoglobinPhysiological values:•Hemoglobin P₅₀≈26 torr•pO₂in lungs ≈100 torr•pO₂in tissues ≈20 torrThis allows hemoglobin to:•Load oxygen efficiently in the lungs•Release oxygen efficiently in tissues

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