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MCAT

Unit 2: Lesson 1

Foundation 1: Biomolecules

Basic concepts in bioenergetics: phosphoryl group transfers and ATP hydrolysis

Problem

Adenosine 5’-triphosphate (ATP) serves as the main source of free energy in living cells. The energy stored by ATP can be liberated through direct hydrolysis or by group transfer.
Figure 1 Molecular structure of ATP, ADP, and AMP
Direct hydrolysis of ATP consists of nucleophilic attack by start text, H, end text, start subscript, 2, end subscript, start text, O, end text at the gamma phosphate position of ATP and cleavage of the gamma, minus, beta phosphoanhydride bond. Direct hydrolysis of ATP liberates energy mainly in the form of heat. This heat energy can be used to cycle proteins through different conformations (for instance, in muscle contraction).
start text, A, T, P, end text, plus, start text, H, end text, start subscript, 2, end subscript, start text, O, end text, right arrow, start text, A, D, P, end text, plus, start text, P, end text, start subscript, start text, i, end text, end subscript
delta, start text, G, apostrophe, end text, degrees, equals, minus, 30, point, 5, start fraction, start text, k, J, end text, divided by, start text, m, o, l, end text, end fraction
Group transfer reactions involve the covalent transfer of a portion of the ATP molecule to a substrate. Table 1 classifies the three group transfer reactions that involve ATP by the phosphate position of ATP at which nucleophilic attack occurs.
Table 1 Group transfer reactions involving ATP
Phosphate position of ATPGroup transferred
Phosphoryl transfergammaminus, start text, P, O, end text, start subscript, 3, end subscript, start superscript, 2, minus, end superscript
Pyrophosphoryl transferbetaminus, start text, P, O, end text, start subscript, 3, end subscript, start text, P, O, end text, start subscript, 3, end subscript, start superscript, 3, minus, end superscript
Adenylyl transferalphaminus, start text, P, O, end text, start subscript, 3, end subscript, start superscript, minus, end superscript, start text, A, d, e, n, o, s, i, n, e, end text
Adenylyl transfer has the largest negative standard free energy change, and is commonly coupled to biological reactions that have a particularly large positive standard free energy change. One example is fatty acid adenylylation, in which exergonic adenylylation of fatty acid (initiating by nucleophilic attack by the carboxylate ion of the fatty acid) and pyrophosphatase-catalyzed pyrophosphate hydrolyzation is coupled to endergonic condensation of fatty acid and coenzyme A, yielding fatty acyl-coA, AMP, and two inorganic phosphate molecules:
start text, f, a, t, t, y, space, a, c, i, d, end text, plus, start text, C, o, A, end text, plus, start text, A, T, P, end text, right arrow, start text, f, a, t, t, y, negative, a, c, y, l, negative, C, o, A, end text, plus, start text, A, M, P, end text, plus, start text, P, end text, start subscript, start text, i, end text, end subscript
delta, start text, G, apostrophe, end text, degrees, equals, minus, 34, start fraction, start text, k, J, end text, divided by, start text, m, o, l, end text, end fraction
Which of the following best describes the difference between exergonic reactions (adenylylation, pyrophosphate hydrolyzation) and endergonic reactions (condensation of fatty acid with coenzyme A)?
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