L17-19 - 1 Biochemistry Sixth Edition Chapter 7 Hemoglobin:...

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Unformatted text preview: 1 Biochemistry Sixth Edition Chapter 7 Hemoglobin: Portrait of a Protein in Action Copyright 2007 by W. H. Freeman and Company Berg Tymoczko Stryer Big BANG prokaryotes eukaryotes Possible Timeline for Biochemical Evolution An aerobic atmosphere allows much more energy to be extracted from metabolites. Thermodynamically, not unlike your basic combustion reaction: C x H y O z + O 2 --> H 2 O + CO 2 + A MAJOR evolutionary event!!! Hemoglobin evolves must have had an ancient function other than O 2 binding Purple disc is O 2-bound heme (Hemoglobin is itself 2 2 ) Red blood cells from lungs to tissues (circulatory system) Getting O 2 to the cells that need it 2 Three review slides follow... Some facts about myoglobin vs hemoglobin Myoglobin Hemoglobin function: binds O 2 reversibly binds O 2 reversibly structure: globin fold globin fold O 2 afFnity: high low # polypeptides: 1 4 cooperativity in O 2 binding: none positive O 2 binding affected by CO 2 , H + , ... no yes Sequence of myoglobin: human ( magenta ), chimp ( blue ) Differ at only one amino acid Human and chimp DNA are 99% similar. Must be evolutionarily related. Evolutionary relationships: Manifest in protein sequence comparisons Sequence alignments are used to detect homology: Identities PLUS conservative substitutions Substitution matrices (previous slide) lead to: Identities (boxed) Conservative substitutions (shaded) 3 Some basic facts about hemoglobin and myoglobin Hemoglobin is present here; Its more ancient than myoglobin! 7.1 Myoglobin and hemoglobin bind oxygen at iron atoms in heme Structure is: largely -helical globular Frst protein to have crystal structure solved: by John Kendrew (1950s) myoglobin was from sperm whale Exists in two forms: 1.) O 2-free: deoxymyoglobin 2.) O 2-bound: oxymyoglobin Living igure! The ability to bind O 2 depends on a bound heme group Accessory groups such as heme are known as prosthetic groups or cofactors . The organic portion of heme ( i.e. all but the e) is called the protoporphyrin . Make sure you review the following and can write their structures: pyrrole, propionate, methyl, vinyl, methine (=CH-). tetrapyrrole towards solution towards hydrophobic core-- 2+ q overall = 2(-1) + 1(+2) = 0 sp 3 sp 2 4 Binding of O 2 to iron ion leads to a change in structure Iron is six-coordinate: The 1st four coordination sites come from Ns in the tetrapyrrole plane. The 5th coordination site is from the far, or , N of a His known as the proximal histidine. The 6th coordination site, present in oxyhemoglobin only, is from the bound oxygen The proximal His O 2 Iron: Out-of-plane In-plane N (Fe 2+ =larger =doesnt t) (Fe 3+-character = smaller=ts!) Binding of O 2 is associated with accepting of an e- from Fe Shown here is a resonance ( i.e . depicts e- distribution)....
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This note was uploaded on 07/23/2008 for the course CHEM 476 taught by Professor Bevilacqua,philip during the Fall '07 term at Pennsylvania State University, University Park.

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L17-19 - 1 Biochemistry Sixth Edition Chapter 7 Hemoglobin:...

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