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Biochemistry I - Oxygen Binding by Myoglobin and Hemoglobin

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    Study Guide Biochemistry I – Oxygen Binding by Myoglobin and Hemoglobin 1. Hemoglobin and Myoglobin Hemoglobin and myoglobin are proteins that bind oxygen, but they are designed for different roles in the body. Although their primary amino acid sequences are only slightly related, the differences between the two proteins are mostly conservative substitutions . More importantly, their secondary structures are almost identical. Both proteins are largely α - helical , and the helices are arranged in a very similar way. 1.1 The Heme Group and Oxygen Binding Both proteins bind oxygen using a heme group . • Each protein binds one O ₂ molecule per heme • The oxygen binds to an iron atom (Fe² ⁺ ) in the heme • The bond between oxygen and iron is a coordinate covalent bond Heme structure: • Square - planar molecule • Made of four pyrrole rings • Nitrogen atoms bind four of iron’s six coordination sites The remaining two coordination positions: 1. One binds to a histidine side chain of the protein → called the proximal histidine 2. One binds oxygen (O ₂ )

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    Study Guide Figure 1: Heme group showing Fe, proximal histidine (F8), distal histidine (E7), and O ₂ binding site 1.2 Role of the Distal Histidine Free heme reacts poorly with oxygen because O ₂ can oxidize Fe² ⁺ to Fe³ ⁺ , which binds oxygen weakly. This would make hemoglobin and myoglobin useless. This problem is prevented by the distal histidine : • It forces oxygen to bind at a bent angle • Prevents formation of a linear Fe – O – O bond • Reduces oxidation of iron • Results in weaker but reversible oxygen binding Weaker binding is actually useful because it allows easy oxygen release . 1.3 Structural Differences Between Hemoglobin and Myoglobin The major functional difference between the two proteins lies in their quaternary structure . Myoglobin • Monomeric (single polypeptide chain) • No quaternary structure

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    Study Guide • Binds oxygen very tightly • Found in muscle cells • Stores oxygen and facilitates its diffusion Hemoglobin • Tetramer • Composed of: o Two α subunits o Two β subunits • Binds oxygen less tightly • Found in red blood cells • Transports oxygen from lungs to tissues This difference in oxygen affinity allows oxygen to move from hemoglobin → myoglobin → muscle cells. 1.4 Oxygen Binding by Myoglobin Myoglobin binds oxygen through a simple equilibrium reaction : Because myoglobin has one heme , it can bind only one oxygen molecule . Oxygen Binding Curve of Myoglobin Oxygen concentration is measured as partial pressure (pO ₂ ) . The binding curve is hyperbolic , described by:

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    Study Guide Where: • Y = fraction of oxygen - bound myoglobin • pO ₂ = oxygen partial pressure (torr) • P ₅₀ = pO ₂ at which 50% of myoglobin is saturated Figure 2: Hyperbolic oxygen - binding curve of myoglobin compared to hemoglobin 1.5 Physiological Meaning of P ₅₀ • Myoglobin P ₅₀ ≈ 1 torr • Atmospheric pO ₂ ≈ 159 torr • Therefore, myoglobin is almost fully saturated at normal oxygen levels Y ≈ 0.99 at pO ₂ = 159 torr In blood: • Venous saturation ≈ 0.91 • Arterial saturation ≈ 0.97 1.6 Oxygen Binding by Hemoglobin Hemoglobin has four heme groups , so oxygen binding occurs in four steps :

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    Study Guide This shows the stepwise binding of oxygen to hemoglobin , up to four O ₂ molecules. Each oxygen - binding event affects the next one. This phenomenon is called cooperativity , a type of homotropic effect . 1.7 Cooperative Binding and Sigmoidal Curve Because of cooperativity: • Hemoglobin’s oxygen - binding curve is sigmoidal (S - shaped) • Not hyperbolic like myoglobin Physiological values: • Hemoglobin P ₅₀ ≈ 26 torr • pO ₂ in lungs ≈ 100 torr • pO ₂ in tissues ≈ 20 torr This allows hemoglobin to: • Load oxygen efficiently in the lungs • Release oxygen efficiently in tissues

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