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The biosynthesis of cell constituents I

The biosynthesis of cell constituents I. Objectives Why is ATP a high-energy phosphate compound? High phosphoryl transfer potential compounds. Energy charge regulates metabolism. Sources of ATP during exercise. Refer to chapter 14, Stryer, 5e and chapter 13, Lehninger, 4e.

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The biosynthesis of cell constituents I

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  1. The biosynthesis of cell constituents I • Objectives • Why is ATP a high-energy phosphate compound? • High phosphoryl transfer potential compounds. • Energy charge regulates metabolism. • Sources of ATP during exercise. Refer to chapter 14, Stryer, 5e and chapter 13, Lehninger, 4e. Lecture 15, Michael Schweizer

  2. The central role of ATP metabolism. ATP is the shared chemical intermediate linking energy-releasing to energy-requiring processes. It’s role in the cell is analogous to that of money in an economy: it is ‘earned/produced’ in exergonic reaction and ‘spent/consumed’ in endergonic reactions.

  3. High energy charge regulates metabolism. High conc. of ATP inhibit the relative rates of a typical ATP-generating (catabolic) pathway and stimulate the typical ATP-utilizing (anabolic) pathway (The energy charge, like the pH of a cell, is ‘buffered’!).

  4. ATP is the universal currency of free energy in biology

  5. Energy from sunlight of from food Energy available for cellular work The interconversion of ATP and ADP occurs in a cycle.

  6. Why is ATP often termed a high energy phosphate (squiggle ~) compound? Bond strain in the reactant caused by electrostatic repulsion (ATP4-; ADP3-; AMP 2-; HPO42-); therefore the formation of ATP is not favoured. The hydrolysis of the terminal phosphate group of ATP removes an unfavourable repulsion between negative charges. Stabilization of the products by resonance (resonance hybrids are more stable; contain less free energy). Stablization of the products by ionization. Stabilisation of the products due to hydration: H2O binds more effectively to ADP and Pi than to the phosphoanhydride part of ATP. Phosphoanhydride bonds have a much larger (negative) standard free energy of hydrolysis than ester linkages because the electrons are attracted to the phosphorous atom. Gain of entropy due to the existence of two reactants Stabilization of the products by isomerization.

  7. Chemical basis for the large free energy change associated with ATP hydrolysis: (1) The charge separation resulting from hydrolysis relieves electrostatic repulsion among the four negative charges on ATP. (2) The product Pi is stabilized by formation of a resonance hybrid in which each of the four P-O bonds has the same degree of double-bond character and the H+ is not permantly associated with anyone of the O atoms. (3) The product ADP2- immediately ionizes, releasing a proton into the medium. (4) A fourth factor (not shown) that favours ATP hydrolysis is the greater degree of solvation of the products Pi and ATP relative to ADP which further stabilizes the products relative to the reactants.

  8. Resonance stabilisation of orthophosphate, HPO42-

  9. Hydrolysis of 1.3-bisphosphoglycerate.

  10. Hydrolysis of phosphocreatine>

  11. Hydrolysis of PEP: this reaction catalyzed by pyruvate kinase is followed by spontaneous tatomerization of the product pyruvate. Tatautomerization is not possible in PEP and the products are stabilized relative to the reactants. Resonance stabilization of Pi also occurs (not shown).

  12. Ranking of biological phosphate compounds by standard free energies of hydrolysis.

  13. Note: In the past biochemists often said that compounds, eg PEP, Cr-P, ATP contain “high-energy phosphate bonds”. This concept is not strictly correct, for there is nothing special about the phosphate bonds in these compounds. The explanation for the free high energies of hydrolysis of some of the substances lies in the special properties of both the reactants and products in the reaction. Factors can contribute to making these free energy changes larger! PEP, ATP, etc are high-energy bonds in the sense that much free energy is released when they are hydrolyzed. The free energy released by hydrolysis of phosphate compounds results from the products of the reactions having a lower free energy content than reactants.

  14. Sources of ATP during exercise. In the initial seconds exercise is powered by existing phosphoryl transfer compounds (PCr, ATP). Subsequently ATP must be regenerated by metabolic pathways (see Stryer, 5e!)

  15. The firefly, a beetle of the Lampyridae family.

  16. Firefly bioluminescence cycle.

  17. ATP hydrolysis in two steps. • The contribution of ATP is often shown as a single step, but it is almost always a two-step process. • Reaction catalyzed by glutamine synthetase. (1) a phosphoryl group is transferred from ATP to glutamate. • (2) The phosphoryl group is displaced by NH3 and released as Pi.

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