21. The Generation of Biochemical Energy

Hydrolysis of Adenosine Diphosphate yields:

An athlete is training for a marathon. Every mile that the athlete runs, an average of 117.5 kcal of energy is expanded. How many moles of ATP would the athlete burn during a full marathon (26.2 mi)? Use conversion factor: 1 mole ATP = 7.3 kcal.

Acetyl phosphate, whose structure is given here, is another compound with a relatively high free energy of hydrolysis.

Using structural formulas, write the equation for the hydrolysis of this phosphate.

A common metabolic strategy is the lack of reactivity—that is, the slowness to react—of compounds whose breakdown is exergonic. For example, hydrolysis of ATP to ADP or adenosine monophosphate (AMP) is exergonic but does not take place without an appropriate enzyme present. Why would the cell use this metabolic strategy?

One of the steps in lipid metabolism is the reaction of glycerol (1,2,3-propanetriol, HOCH₂CH(OH)CH₂OH, with ATP to yield glycerol 1-phosphate. Write the equation for this reaction using the curved arrow symbolism.

The hydrolysis of acetyl phosphate to give acetate and hydrogen phosphate ion has ∆G = -10.3 kcal/mol (-43.1 kJ/mol). Combine the equations and ∆G values to determine whether coupling of this reaction with phosphorylation of ADP to produce ATP is favorable. (You need give only compound names or abbreviations in the equations.)

For the first step in fatty acid catabolism, we say that ATP is used to “drive” the reaction that links the fatty acid with coenzyme-A. Without ATP hydrolysis, would you predict that the linking of fatty acid to coenzyme-A would be ­exergonic or endergonic? In fatty acid CoA synthesis, the hydrolysis of the ATP portion is based on what major strategy of metabolism?

Why is ATP sometimes called a high-energy molecule?

What general kind of chemical reaction does ATP participate in?

What does it mean when we say that two reactions are coupled?

Write the reaction for the hydrolysis of 1,3-bisphosphoglycerate coupled to the phosphorylation of ADP using the curved-arrow symbolism.

The following reactions occur during the catabolism of acetyl-CoA. Which are exergonic? Which is endergonic? Which reaction produces a phosphate that later yields energy by giving up a phosphate group?

c. L-Malate + NAD⁺ → Oxaloacetate + NADH + H⁺

∆G = +17 kcal/mol (+129.3 kJ/mol)

Why is ATP considered an energy-rich compound?

One cell at work may break down 2 million (2 000 000) ATP molecules in one second. Some researchers estimate that the human body has about 10¹³ cells.

b. If ATP has a molar mass of 507 g/mole, how many grams of ATP are hydrolyzed in one day?

List the energy yield in ATP molecules for each of the following:

a. NADH → NAD⁺