metabolism_lecture_3 - Payoffphase Stage2 The payoff phase...

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Stage 2 Payoff phase
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The payoff phase starts: Glyceraldehyde 3-phosphate dehydrogenase Energetics: a downhill reaction ( oxidation ) is  coupled to the uphill reaction of forming  a very high-energy  acyl phosphate 6
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Critical  cysteine thiohemiacetal Oxid. thioester acid anhydride Mechanism of GAP dehydrogenase: high-energy thioester  When arsenate (As   3 O   4    3- ) instead of P   i  is present: Leads to “uncoupling” of oxidation and phosphorylation. Acyl arsenate hydrolyzes spontaneously -> loss of high-energy compound, no ATP formation in next step P i Phosphorylytic  cleavage From Stryer, 4 th  edition
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What makes thioesters high-energy intermediates compared to oxygen esters? Possible resonance stabilization of oxygen esters  lowers their free energy. Thus, lower  G for hydrolysis to acid + alcohol  Krebs cycle
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Phosphoglycerate kinase: First  formation of ATP in glycolysis reversible   spontaneous forward reaction (cp. high-energy acid anhydride) ADP + Pi -> ATP    G’’ 0  = 30.5 kJ/mol BPG -> 3-PG + P i     G’’ 0  =  -49  kJ/mol 7
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Phosphoglycerate mutase: Enzymes that shift functional groups  around are called  mutases. Similar mechanism for phosphoglucomutase, which regulates entry into glycolysis in muscle (see later)  Covalent catalysis  Enzyme contains  phosphohistidine 8 Cofactor-independent ping-pong mechanism
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9 Enolase: Dehydration increases hydrolysis potential of the phosphate  (better phosphoryl-donor). Dephosphorylation enol tautomerizes to keton (secondary alcohol) G’ 0  = - 17.9 kJ/mol G’ 0  = - 61.9 kJ/mol
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10 Pyruvate kinase: subsequent keto-enol tautomerization strongly drives the reaction (strongly increases the phosphoryl transfer potential of PEP) Regulation: Pyruvate kinase requires divalent metals ( Mg ++  or Mn ++ ) for activity. Under physiological conditions, the  activity  of PK  is limited by [Mg ++ ]. At high [ATP], 
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This note was uploaded on 12/03/2009 for the course MCB 58168 taught by Professor Thorner during the Fall '09 term at University of California, Berkeley.

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metabolism_lecture_3 - Payoffphase Stage2 The payoff phase...

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