Biology

HEMOGLOBIN AND MYOGLOBIN

OXYGEN CARRIERS A. Why do we need oxygen carriers? i. Cannot carry enough in blood to meet metabolic demand ii. Oxygen is very reactive – oxidizes iii. Oxygen cannot diffuse very easily (we have thick skin) B. Properties of a good oxygen carrier i. Binds oxygen at a high [O2] ii. Doesn’t oxidize cellular components iii. Gives up oxygen on demand C. Hemoglobin and Myoglobin i. Cooperativity 1. Hemoglobin needs to have high affinity to bind O2 in the lungs, but low affinity to unload to myglobin 2. Sigmoidal curve: represents weak-binding state at low P02 and strongbinding state at high P02 ii. Hemes 1. The heme binds O2, not the protein 2. Function of protein: provides crevice – keeps heme from oxidizing a. absence of protein: ferrous atom (Fe2+)  ferric state (Fe3+) b. heme buried in hydrophilic environment of protein: O2 binding does not result in oxidation 3. Heme structure a. Each polypeptide of protein is made from 8 residues  6 helices – A, B, C, D, E, F b. Fe2+ has 6 coordinating bonds i. 4 bonds = nitrogens from tetrapyrrole ring system ii. 5th bond - Helix F binds to Fe at terminal Histidine th * 5 bond = helix F8, residue 93 in Mb, molecule (His F8)* = proximal histidine residue 92 in the β-chain of Hb and iii. 6th bond – deoxygenated: empty, histidine residue from residue 87 in α-chain of Hb th helix E** hovers; oxygenated: oxygen bonds here ** 6 bond = helix E7, His 64 for Mb, His 63 in b-chain and 58 in a-chain c. Oxygen binds to Fe at 120° angle  easily removed II. MYOGLOBIN A. Physico-chemical properties i. 153 amino acids – single polypeptide chain ii. Very compact: globular structure  little empty space for solution to get in iii. Tertiary structure: 8 alpha helices (A-H), 4 helices terminated by proline residues iv. About 75% is in alpha helical structure v. Polar side chains on outside of protein  interact with solution vi. Myoglobin = storage protein  mainly in skeletal muscle vii. High O2 affinity – does not...