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Published byCori Monica Dickerson Modified over 9 years ago
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Is ubiquitination always the result of mistakes?
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The N-end Rule: The N-terminal amino acid determines half-life Destabilizing N-termini are recognized by a special E2/E3
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The eukaryotic cell cycle is controlled by the ubiquitin pathway
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During the cells cycle synthesis or mitosis DNA damage signals cell cycle arrest. The Mdm2 E2/E3 keeps p53 abundance low under normal conditions. After DNA damage p53 is stabilized and it causes the trancription of a CDK inhibitor, thereby stopping the cell cycle.
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Let’s examine a real world example: The globins
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Oxygen Carriers Hemerythrin Hemocyanin Globins
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Need metals to bind to oxygen…..why? Oxygen is a diradical It has 2 unpaired electrons 1/2 3 O 2 + 1 X ---> 1 XO
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The spin restriction limits the chemical reactivity by imposing a kinetic barrier This is the oxygen paradox Singlet oxygen in the excited state is extraordinarily reactive This is the basis for photodynamic therapy
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Fe(II)-O 2 Fe(III)-O 2 - Fe(III)-O 2 - + Fe(II)Fe(III)-O 2 2- -Fe(III) 2Fe(IV)=OFe(III)-O-Fe(III) 2Fe(IV)=O Metals cause oxygen to become reactive because they are radicals themselves. They eliminate spin restrictions Highly reactive!
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A picket-fence Fe(II)–porphyrin complex with bound O 2 - Metals, along with proteins, can harness the reactivity of oxygen by activating it an shielding it
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Fe(II) binds dioxygen Fe(III) does not Why? Oxygen to metal charge transfer Fe(II)-O 2 Fe(III)-O 2 - Fe(II) will also bind NO, CO, S 2-, CN - Fe(III)-O 2 Fe(IV)-O 2 - Stable Unstable
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The visible absorption spectra of oxygenated and deoxygenated hemoglobins.
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Distal Proximal
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N-terminus C-terminus
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Fractional saturation of myoglobin with oxygen
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Hemoglobin binds oxygen cooperatively This means that the binding of one oxygen to one subunit affects the binding to another subunit
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Deoxy or T state Oxy or R state The two state model of hemoglobin binding
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Major Structural differences upon oxidation of hemoglobin Fe moves from 0.55Å out of the heme plane to 0.22Å out of the plane Extensive 1- 1 contacts unchanged Minimal 1- 2 contact altered by as much as 6 Å 15º offcenter rotation of the protomers
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High spin O h Fe 2+ xy xzyz x 2 -y 2 z2z2 Increased radius Low spin O h Fe 3+ xy xzyz x 2 -y 2 z2z2 Decreased radius
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1) Intra- subunit His-Asp pair 2) Lys- -C-terminus pair 3) Inter- subunit Arg-Asp/C-terminus-Lys pairs 4) Inter- subunit N-terminus-C-terminus pair Ion pairs that stabilize the T-state
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Low pH stabilizes the T state. How? High CO 2 in tissues decreases the pH: the Bohr effect CO 2 + H 2 O ---> H + + HCO 3 -
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Lys- -C-terminus pair Intra- subunit His-Asp pair At low pH His 146 is protonated allowing the ion pair to form
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COO- R-NH 2 + CO 2 R-NH-COO - + H + Carbaminohemoglobin -amino terminus Inter- subunit Arg-Asp/C-terminus-Lys pairs
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deoxyHb can also bind chloride ion tightly High Cl - will cause O 2 release Cl - is higher in veins than in arteries Inter- subunit Arg-Asp/C-terminus-Lys pairs
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Thus the T state is stabilized by: Low pH High CO 2 High Cl -
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Comparison of the O 2 -dissociation curves of “stripped” Hb and whole blood in 0.01M NaCl at pH 7.0.
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2,3-bisphosphoglycerate binds deoxyHb BPG Keeps Hb deoxygenated
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Binding of BPG to deoxyHb.
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The effect of high-altitude exposure on the p 50 and the BPG concentration of blood in sea level– adapted individuals.
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Notice: 8 mM BPG results in less saturation at high altitude….but….results in equivalent release of O 2. Note 38% release of O 2 at sea level with 5 mM BPG and 30% release at high altitude with 5 mM BPG. Also note 37% release at high altitude with 8 mM BPG!
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Fetal hemoglobin ( 2 2 ) Adult hemoglobin ( 2 2 ) Neonatal hemoglobin ( 2 2 ) 1% adult hemoglobin ( 2 2 ) Why are there different globins?
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Myoglobin has a higher affinity for O 2 in tissues
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Fetal hemoglobin ( 2 2 ) No affinity for BPG Thus it will look more like myoglobin
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