Thus, ATP transfers phosphate, NADPH transfers electrons and hydrogen, and acetyl CoA transfers two-carbon acetyl groups. FADH2 (reduced flavin adenine dinucleotide) is used like NADH in electron and proton transfers (Figure 2–39). The reactions of other activated carrier molecules involve the transfer of a methyl, carboxyl, or glucose group for biosyntheses (Table 2–3). These activated carriers
Figure 2–38 The structure of the important activated carrier molecule acetyl Coa. a ball-and-stick model is shown above the structure. The sulfur atom (yellow) forms a thioester bond to acetate. Because this is a high-energy linkage, releasing a large amount of free energy when it is hydrolyzed, the acetate molecule can be readily transferred to other molecules.
Figure 2–39 FaDH2 is a carrier of hydrogens and high-energy electrons, like NaDH and NaDPH. (a) structure of faDh2, with its hydrogen-carrying atoms highlighted in yellow. (B) The formation of faDh2 from faD.
are generated in reactions that are coupled to ATP hydrolysis, as in the example in Figure 2–40. Therefore, the energy that enables their groups to be used for biosynthesis ultimately comes from the catabolic reactions that generate ATP. Similar processes occur in the synthesis of the very large molecules of the cell—the nucleic acids, proteins, and polysaccharides—that we discuss next.
The synthesis of Biological polymers is Driven by aTp hydrolysis
As discussed previously, the macromolecules of the cell constitute most of its dry mass (see Figure 2–7). These molecules are made from subunits (or monomers) that are linked together in a condensation reaction, in which the constituents of a water molecule (OH plus H) are removed from the two reactants. Consequently, the reverse reaction—the breakdown of all three types of polymers—occurs by the enzyme-catalyzed addition of water (hydrolysis). This hydrolysis reaction is energetically favorable, whereas the biosynthetic reactions require an energy input (see Figure 2–9).
The nucleic acids (DNA and RNA), proteins, and polysaccharides are all polymers that are produced by the repeated addition of a monomer onto one end of a growing chain. The synthesis reactions for these three types of macromolecules are outlined in Figure 2–41. As indicated, the condensation step in each case depends on energy from nucleoside triphosphate hydrolysis. And yet, except for the nucleic acids, there are no phosphate groups left in the final product molecules. How are the reactions that release the energy of ATP hydrolysis coupled to polymer synthesis?
For each type of macromolecule, an enzyme-catalyzed pathway exists which resembles that discussed previously for the synthesis of the amino acid glutamine (see Figure 2–35). The principle is exactly the same, in that the –OH group that will
Figure 2–40 a carboxyl group-transfer reaction using an activated carrier molecule. Carboxylated biotin is used by the enzyme pyruvate carboxylase to transfer a carboxyl group in the production of oxaloacetate, a molecule needed for the citric acid cycle. The acceptor molecule for this group-transfer reaction is pyruvate. other enzymes use biotin, a B-complex vitamin, to transfer carboxyl groups to other acceptor molecules. note that synthesis of carboxylated biotin requires energy that is derived from aTp—a general feature of many activated carriers.