Test 2 Review--Biochem369

Test 2 Review--Biochem369 - Test 2 Chapter 4 cont'd 1....

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Test 2 Chapter 4 cont’d 1. Understand how O2 binds to myoglobin a. Heme group—prothetic group which is wedged into a hydrophobic pocket i. Alone heme not effective carrier b/c the central Fe(II) atom is easily oxidized, which cant bind O2 ii. Protein around heme keeps it in right environment iii. Carboxylate facing outward b. Oxygen binds to form the 6 th coordination bond i. Residue His E7 forms a h-bond to oxygen c. Oxygen binding depends on oxygen conc. i. The proportion of total myoglobin that have bound O2 is called the fractional saturation (Y) ii. O2 concentration expressed as p02 for partial pressure of oxygen iii. The relationship btwn (Y) and (pO2) forms a rectangular hyperbola 1. when line flattens out myoglobin is saturated with oxygen 2. The oxygen conc at which myoglobin is ½ saturated (p50) po2=K=2.8 torr (Y=0.5) 2. Understand advantages of quaternary structure a. Hemoglobin (heterotetramer) – contains 2 alpha helices and 2 beta sheets b. Advantages of multi-subunit protein structure i. Cell can construct extremely large proteins 1. asset for large proteins which have to be synthesized piece by piece or outside the celll ii. impact of inevitable errors in transcription and translation can be minimized if affected polypeptide is small and readily replaced iii. interaction between subunits affords opportunity for subunits to influence each others behavior 3. Myoglobin and Hemoglobin a. 4 polypeptide subunits of hemoglobin called a globin—looks like myoglobin i. The two have very similar tertiary structure ii. Both have heme group in a pocket, a His F8 that ligands the Fe(II), and a His E7 that forms an h-bond to O2 b. Amino acids sequences of the three globins (alpha and beta of hemo and myo) are only slightly similar i. Highlights important principle: certain tertiary structures can accommodate a variety of AA sequences; many proteins with unrelated sequences adopt similar structures c. Globins are homologous proteins—have evolved from a common ancestor through mutation i. Primordial globin 4. Oxygen binding to hemoglobin a. Relationship is sigmoidal; why? i. Low O2 conc reluctant to bind first O2 ii. As pO2 increases, O2 binding increase to almost full saturation iii. Binding of 1 st O2 increases the affinity of other O2 binding sites 1. Hemo. 4 heme groups communicate w/ each other to work in a unified fashion --- cooperative binding behavior b. Difference in oxygen affinity btwn hemoglobin and myoglobin ensures that O2 bound to hemoglobin in the lungs is released to myoglobin in the muscles c. Hemoglobin’s overall affinity is lower than that of myoglobin 5. Cooperative behavior a. Four globin subunits undergo conformational change when they bind O2
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i. In deoxyhemoglobin [T-state] (w/o bound O2) the heme Fe has 5 ligands which gives the porphyrin ring a somewhat dome-shaped ii. In oxyhemoglobin [R-state] (O2 binds) Fe has 6 ligands and the Fe moves to center of porphyrin ring 1. movement pull His F8 further toward heme group, and in turn, drags entire F helix so
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Test 2 Review--Biochem369 - Test 2 Chapter 4 cont'd 1....

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