Adenosine Triphosphate as an Energy Carrier
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Reactions or processes that have a large positive ∆G, such as moving ions against a concentration gradient across a cell membrane, are made possible by coupling the endergonic movement of ions with a second, spontaneous process with a large negative ∆G such as the exergonic hydrolysis of adenosine triphosphate (ATP).
ADENOSINE TRIPHOSPHATE AS AN ENERGY CARRIER
Reactions or processes
that have a large positive ∆G, such as moving ions against a concentration
gradient across a cell membrane, are made possible by coupling the endergonic
movement of ions with a second, spontaneous process with a large negative ∆G
such as the exergonic hydrolysis of adenosine triphosphate (ATP). [Note: In the
absence of enzymes, ATP is a stable molecule because its hydrolysis has a high
activation energy.] Figure 6.4 shows a mechanical model of energy coupling. The
simplest example of energy coupling in biologic reactions occurs when the
energy-requiring and the energy-yielding reactions share a common intermediate.
Figure 6.4 Mechanical model of coupling of favorable and unfavorable processes.A Gear with weight attached spontaneously turns in the direction that achieves the lowest energy state. B The reverse movement is energetically unfavorable (not spontaneous). C The energetically favorable movement can drive the unfavorable one.
A. Common intermediates
Two chemical reactions
have a common intermediate when they occur sequentially so that the product of
the first reaction is a substrate for the second. For example, given the
reactions
A + B → C + D
D + X → Y + Z
D is the common intermediate and can serve as a carrier of chemical energy between the two reactions. Many coupled reactions use ATP to generate a common intermediate. These reactions may involve the transfer of a phosphate group from ATP to another molecule. Other reactions involve the transfer of phosphate from an energy-rich intermediate to adenosine diphosphate (ADP), forming ATP.
B. Energy carried by adenosine triphosphate
ATP consists of a
molecule of adenosine (adenine + ribose) to which three phosphate groups are
attached (Figure 6.5). If one phosphate is removed, ADP is produced. If two
phosphates are removed, adenosine monophosphate (AMP) results. The standard
free energy of hydrolysis of ATP, ∆Go, is approximately –7.3
kcal/mol for each of the two terminal phosphate groups. Because of this large
negative ∆Go, ATP is called a high-energy phosphate compound.
Figure 6.5 Adenosine triphosphate.
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