Quiz 2 Study Guide: Hemoglobin & Myoglobin, The Cytoskeleton, Reaction Rates

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University of Massachusetts Amherst
Biochemistry & Molecular Bio.
David Gross

BIOCHEM 523 QUIZ 2 STUDY GUIDE FALL 2013 Here are some guidelines as to what I would like you to know for Quiz 2. I’ve been a little more explicit than for Quiz 1, hence the longer list. Chapter 5 •Myoglobin (single subunit, muscle) Heme: nature of coordination bonds between Fe and porphyrin nitrogens • Has 6 possible coordination bonds • 4 N atoms of porphyrin ring coordinate bond with heme Oxygen binding: fractional saturation (Y) and oxygen partial pressure (pO 2 • The proportion of total myoglobin molecules that have bound O2 is the fractional saturation (Y) • Y=[MbO2]/([Mb]+[MbO2]) (Mb = myoglobin) • pO2 is the partial pressure of oxygen (torr) • Y=pO2/(K+pO2) (K=([Mb][O2])/[MbO2]) • The amount of O2 bound to myoglobin is a function of the oxygen concentration and the affinity of myogloving for O2 Oxygen binding curve: hyperbolic • As O2 concentration increases more and more O2 molecules bind to the heme groups until at very high O2 concentrations virtually all heme groups have been bound • Changes in Mb structure upon oxygen binding • A His residue on the E7 alpha-helix forms a H-bond w/ O2 when it binds •Hemoglobin (heterotetramer with two different chains; red blood cells) Evolutionary relationship of Hb and Mb • Looks a lot like myoglobin • Hemoglobin alpha- & beta-chains and myoglobin have similar tertiary structures • All have heme group in hydrophobic pocket, a His F8 that ligands the Fe(II) ion, and a His E7 that forms a H-bond with O2 • The A.A. sequence is only 18% similar between the 3 however • Invariant residues are those that are identical in all the glovins and are essential for the structure and function (can’t be replaced) • Conservatively substituted areas are under less selective pressure and A.A.’s can be switched out by a similar A.A. • Other positions are viriable and can accommodate a variety of residues Oxygen binding • Binding is cooperative, binding of one O2 molecule increases the affinity for another O2 molecule • @ low O2 concentrations hemoglobin is reluctant to bind the first O2 molecule • Hemoglobin’s four heme groups are not independent but communicate with each other in order to work in a unified fashion Oxygen binding curve: sigmoidal • @ low O2 concentrations the affinity of hemoglobin for O2 is low, as O2 concentration increases the saturation of hemoglobin increases sharply indicating cooperative binding, until virtually all hemoglobin molecules have been bound by O2 • Cooperative binding of oxygens to the four subunits • The 4 O2 taken up by hemoglobin binds with about 100X greater affinity than the first • In lungs where pO2 is high, hemoglobin is very highly saturated, the pO2 in tissue is less, hemoglobin is not highly bound to O2, then myoglobin takes up O2 and distributes it • In deoxyhemoglobin the heme Fe ion has 5 ligands, the porphyrin ring is dome- shaped, Fe lies bowed slightly toward His F8 • Oxyhemoglobin has 6 ligands, moves into the center of the porphyrin plane, pulls His F8 toward the heme group, drags entire F helix • The entire protein must move for this to happen, results in rotation of one alpha- beta unit relative to the other • Hemoglobin has 2 quaternary structures, deoxyhemoglobin is “tense” (T), oxyhemoglobin is “relaxed” (R) • Whenn one molecule binds, subsequent molecules bind easier because hemoglobin is already in the R conformation Regulation of oxygen binding Bohr Effect • Normally when O2 binds to hemoglobin H+ is released, decreasing pH • Therefore increasing the pH of a solution of hemoglobin favors O2 binding and pushes the reaction to the right • Decreasing the pH favors O2 dissociation and pushes the reaction to the left • The reduction of hemoglobin’s oxygen-binding affinity when the pH decreases is known as the Bohr effect Role of 2,3,-bisphoisphoglycerate • Binds in central cavity of hemoglobin, only in T state • 5 negative charges on BPG interact w/ positively charged groups in deoxyhemoglobin • The presence of BPG stabilizes the deoxy conformation • W/o BPG hemoglobin would bind O2 too tightly to release it to cells Fetal hemobglobin differs slightly from adult hemoglobin in structure; higher O 2ffinity • has the composition alpha2,gamma2 • One of the His residues that binds to BPG not pressent in gamma strand, decreased BPG binding • Hemoglobin has a higher O2 affinity than adult hemoglobin for O2, helps transfer O2 from maternal circulation across placenta to fetus •Nature and importance of cytoskeleton • Formed from microfilaments (actin), microtubules (tubulin), and intermediate filaments (collagen) • Collagen fibers provide extracellular support in multicellular organisms • Microfilaments and microtubules also play a role in motion of cell •Pattern of assembly of actin into microfilaments • Globular protein • Polymerized actin called F-actin, monomeric globular form called G-actin • Each actin subunit has same orientation,fibers have distince polarity, end w/ ATP site = - end, opposite end = + end • subunits polymerize in a right handed helix role of ATP in polymerization of actin monomers • most of actin subunits in a microfilament contain bound ADP b/c the polymerization rxn is catalyzed by F-actin b
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