1. Chapter 13.3: ATP CHEM 7784 Biochemistry Professor Bensley

2. CHAPTER 13.3 Phosphoryl Group Transfers and ATP –Explore biomolecules that are “high energy” with respect to their hydrolysis and group transfers Today’s Objectives: To

3. ATP

4. Phosphoryl Transfer from ATP ATP is frequently the donor of the phosphate in the biosynthesis of phosphate esters

6. Hydrolysis of ATP is Favorable Under Standard Conditions Better charge separation in products Better solvation of products More favorable resonance stabilization of products

8. Actual  G of ATP Hydrolysis Differs from  G’° The actual free energy change in a process depends on –The standard free energy –The actual concentrations of reactants and products The free energy change is more favorable if the reactant’s concentration exceeds its equilibrium concentration True reactant and the product are Mg-ATP and Mg-ADP, respectively  G 0 also Mg ++ dependent

10. Actual ATP Concentration Depends on Tissue Type Cellular ATP concentration is usually far above the equilibrium concentration, making ATP a very potent source of chemical energy

11. Several Phosphorylated Compounds Have Large  G’° for Hydrolysis Again, electrostatic repulsion within the reactant molecule is relieved The products are stabilized via resonance, or by more favorable solvation The product undergoes further tautomerization

12. Hydrolysis of Thioesters Hydrolysis of thioesters, such as acetyl-CoA is strongly favorable Acetyl-CoA is an important donor of acyl groups –Feeding two-carbon units into metabolic pathways –Synthesis of fatty acids In acyl transfers, molecules other than water accept the acyl group

14. In Summary For hydrolysis reactions with large, negative, standard free-energy changes, products are more stable than reactants if: 1.Bond strain in reactants due to electrostatic repulsion is relieved by charge separation 2.Products are stabilized by ionization as for ATP, acyl phosphates, and thioesters 3.Products are stabilized by isomerization (PEP) 4.Products are stabilized by resonance

15. Chapter 13: Summary Group transfer reactions are favorable when the free energy of products is much lower than the free energy of reactants. In biochemical phosphoryl transfer reactions, the good phosphate donors are destabilized by electrostatic repulsion, and the reaction products are often stabilized by resonance. Unfavorable reactions can be made possible by chemically coupling a highly favorable reaction to the unfavorable reaction. For example, ATP can be synthesized in the cell using energy in phosphoenolpyruvate. Oxidation-reduction reaction commonly involve transfer of electrons from reduced organic compounds to specialized redox cofactors. The reduced cofactors can be used in the biosynthesis, or may serve as a source of energy for ATP synthesis. In this chapter, we learned that the rules of thermodynamics, and organic chemistry still apply to living systems. For example: