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Types of Work Performed by ATP, ATP Hydrolysis and Free Energy.
Typology: Lecture notes
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Thousands of reactions take place in living cells. Many reactions require the addi- tion of energy for the assembly of complex molecules from simple reactants. These reactions include DNA synthesis, protein synthesis, and the construction of cell walls and other cellular structures. Other cell-driven actions—such as muscle contractions in animals, the motion of flagella in bacteria ( Figure 1 ), and the movement of sap within a tree—also require a supply of energy. Since so many cellular functions require energy, cells need a constant supply of energy. Even though the species on Earth are very diverse, all cells in every organism use the same energy carrier for almost all of their energy-driven actions. This energy comes in the form of a compound called adenosine triphosphate, or ATP. ATP directly supplies the energy that powers nearly every cellular function, and it is considered the universal energy “currency.” The types of work that are carried out by ATP include mechanical, transport, and chemical work ( Table 1 ).
Table 1 Types of Work Performed by ATP
Mechanical work Transport work Chemical work
Figure 1 A bacterium with flagella
Adenosine triphosphate (ATP) consists of three parts: a nitrogenous base called adenine, which is linked to a five-carbon sugar called ribose, which in turn is linked to a chain of three phosphate groups ( Figure 2 , next page ). ATP contains large amounts of free energy. The energy of the molecule is high because of its three nega- tively charged phosphate groups. The phosphate groups crowd together, and their close proximity creates a mutual repulsion of their electrons. The mutual repulsion contributes to the weakness of the bond holding the groups together. The bonds of ATP are easily broken by a catalyzed reaction with water—a process called hydrolysis (Figure 2). The hydrolysis reaction results in the breaking off of the end (or terminal) phosphate group and the formation of two products—adenosine diphosphate (ADP) and an inorganic phosphate (Pi). In addition, an H^1 ion is released into the solution. As bonds in these new products form, free energy is released.
ATP 1 H 2 O S^ ADP 1 Pi ∆G 5 – 30.5 kJ/mol
Note that the H+^ ion is not normally shown in the chemical equation, since it is understood to be associated with the formation of Pi. Recall, from Section 3.1, that during a chemical reaction, bonds in the reactants break and new bonds in the products form. During ATP hydrolysis, bonds form when a new —OH group attaches to the phosphorus atom of the phosphate group and when an electron attaches to the oxygen that remains on the ADP molecule. Energy is also released as the H+^ ion interacts with water molecules. The bond rearrangements and the change in entropy result in an overall free energy change of – 30.5 kJ/mol. When ATP splits into ADP and Pi within a cell, the phosphate group, rather than remaining free in solution, often becomes attached to another molecule, which results in a dif- ferent bonding arrangement.
nEL^ 3.2^ atP: Energy Currency of the Cell^141
In a process called energy coupling (Section 3.1), ATP can be moved into close con- tact with a reactant molecule of an endergonic reaction. Then, during the reaction, the terminal phosphate group breaks away from the ATP and transfers to the reactant molecule. Attaching a phosphate group to another organic molecule is a process called phosphorylation. It results in the molecule gaining free energy and becoming more reac- tive. Energy coupling requires an enzyme to bring the ATP molecule close to the reactant molecule of the endergonic reaction. There are specific sites on the enzyme that bind both the ATP molecule and the reactant molecule. In this way, the two molecules are brought close to one another, and the transfer of the phosphate group takes place. Most of the work carried out in a cell is dependent on phosphorylation for energy. An example of energy coupling that is common to most cells is the reaction in which ammonia, NH 3 , is added to glutamic acid ( Figure 3(a) , next page). The resulting product of this reaction is glutamine, which is an amino acid. The reaction can be written as follows: glutamic acid 1 ammonia S^ glutamine 1 H 2 O ∆G 5 1 14.2 kJ/mol The glutamine that is produced takes part in the assembly of proteins during pro- tein synthesis. The positive value of ∆G shows that the reaction is endergonic and cannot proceed spontaneously. Therefore, the coupling of this reaction with ATP hydrolysis gives it the necessary energy to proceed. As a first step, the phosphate group is removed from the ATP and transferred to the glutamic acid molecule, forming glutamyl phosphate ( Figure 3(b) , next page): glutamic acid 1 ATP S^ glutamyl phosphate 1 ADP ∆G , 0 The change in free energy, ∆G, is negative for this reaction. This means that the reac- tion is exergonic and can proceed spontaneously. In the second step of the reaction, glutamyl phosphate reacts with ammonia: glutamyl phosphate 1 ammonia S^ glutamine 1 Pi (inorganic phosphate) ∆G , 0
phosphorylation the transfer of a phosphate group, usually from ATP, to another molecule
P (^) i ADP
energy
O –^ P
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O P
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O P O
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C OH
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sugar (ribose)
nucleotide base (adenine)
H2O
C03-F13-OB12USB Crowle Art Group Deborah Crowle 2nd pass
Biology 12
FN CO
Pass Approved Not Approved
three phosphate groups
Figure 2 ATP releases large amounts of free energy during a hydrolysis reaction as new bonds form in the products. The reaction results in the addition of a new OH group to a released phosphate as well as the addition of an electron to the terminal oxygen on the ADP and the release of an H+^ ion into solution.
142 Chapter 3 • an introduction to Metabolism (^) nEL
There is very little ATP in our diet, and yet we require it as a constant energy source within our cells. ATP hydrolysis releases energy obtained from the foods we eat, and then more energy from food is used to reassemble the ATP. If cells constantly need to use energy from food to reassemble ATP, then why do cells use ATP as their energy currency to begin with? It would seem that cells could just use the food molecules directly as sources of energy. Cells use ATP as an immediate source of energy because it has specific properties that are important for the biochemical reactions that allow proper cell functioning. ATP provides a manageable amount of energy, and couples in very similar ways to thousands of different reactions in our cells. This ability to couple to so many different endergonic reactions gives ATP its “universal” characteristic. The widespread use of ATP in all living things is an evolved characteristic. Many other molecules are energy rich, but they vary in size and shape, in the amount of energy they release, and in the types of reactions to which they can couple. Also, the availability of these molecules is not always reliable or predictable. If a certain reaction required the use of a par- ticular food molecule—for example, glucose—as an energy source to drive a coupled reaction, and the cell did not have any glucose, it could not power the given reaction, even if there were other energy-rich molecules in the cell. The ability to assemble ATP using the energy from a variety of food molecules ensures that all vital reactions in the cell can be performed. Complex food molecules also require numerous reactions to release their energy, but ATP can be created and accessed immediately. The only requirement is that at least one of these food sources is available for generating ATP. Although ATP is the energy currency of cells and is the immediate source of energy to drive cellular processes, it is not the only energy carrier in cells. There are other phosphate carriers, such as guanosine triphosphate (GTP), that are used specifically as carriers of high-energy electrons. In this section, you learned that cells can use ATP as a source of energy to drive endergonic reactions. There are, however, other factors that influence a cell’s ability to perform endergonic and even exergonic reactions. Critical among these factors is the need to overcome the activation energy requirements for a particular reaction (Section 3.1). If the activation energy requirement is very high because the bonds in the reactants are very strong, a reaction will not start even if it is exergonic overall. As you will learn in the next section, some proteins called enzymes play an important role in lowering the activation energy for certain reactions.
Figure 4 (^) The ATP cycle couples reactions that release free energy (exergonic) to reactions that require free energy (endergonic).
energy
Exergonic reactions supply energy for endergonic reaction producing ATP.
Exergonic reaction hydrolyzing ATP provides energy for endergonic reactions in the cell.
energy
ADP
ATP/ADP cycle
H 2 O H 2 O
P (^) i
ATP
ATP cycle the cyclic and ongoing breakdown and re-synthesis of ATP
cycle operates at an astonishing rate. In fact, if ATP were not formed in the cell from ADP and Pi, the average human would need about 75 kg of ATP per day. The con- tinued breakdown and resynthesis of ATP is a process called the ATP cycle ( Figure 4).
144 Chapter 3 • an introduction to Metabolism (^) nEL
3.2 Review
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Figure 5
nEL^ 3.2^ atP: Energy Currency of the Cell^145