Figure 2–41 The synthesis of polysaccharides, proteins, and nucleic acids. synthesis of each kind of biological polymer involves the loss of water in a condensation reaction. not shown is the consumption of high-energy nucleoside triphosphates that is required to activate each monomer before its addition. in contrast, the reverse reaction—the breakdown CH2OHOHOOHOHCH2OHOHOOHOHCH2OHOOHOHOOCH2OHOOHOHCH2OHOOHOHOOCH2OHOOHOHOHOOH(A)POLYSACCHARIDESglucoseglycogenglycogenH2O energy from nucleoside triphosphate hydrolysis HCROCNHHCRCOOHHHRHCCNOOHHCROCNHHCRCRHCCNOOHHOprotein amino acid (C) PROTEINS A OCH2OHOOOO_POC OCH2OHOHOHOPOG OOHOHCH2O_A OCH2OHOOOO_POC OCH2OHOOPOG OOHOHCH2O_H2O (B) NUCLEIC ACIDS RNA nucleotide H2O energy from nucleoside triphosphate hydrolysis energy from nucleoside triphosphate hydrolysis RNA protein of all three types of polymers—occurs by the simple addition of water (hydrolysis).
be removed in the condensation reaction is first activated by becoming involved in a high-energy linkage to a second molecule. However, the actual mechanisms used to link ATP hydrolysis to the synthesis of proteins and polysaccharides are more complex than that used for glutamine synthesis, since a series of high-energy intermediates is required to generate the final high-energy bond that is broken during the condensation step (discussed in Chapter 6 for protein synthesis).
Each activated carrier has limits in its ability to drive a biosynthetic reaction. The ∆G for the hydrolysis of ATP to ADP and inorganic phosphate (Pi) depends on the concentrations of all of the reactants, but under the usual conditions in a cell it is between –46 and –54 kJ/mole. In principle, this hydrolysis reaction could drive an unfavorable reaction with a ∆G of, perhaps, +40 kJ/mole, provided that a suitable reaction path is available. For some biosynthetic reactions, however, even –50 kJ/mole does not provide enough of a driving force. In these cases, the path of ATP hydrolysis can be altered so that it initially produces AMP and pyrophosphate (PPi), which is itself then hydrolyzed in a subsequent step (Figure 2–42). The whole process makes available a total free-energy change of about –100 kJ/ mole. An important type of biosynthetic reaction that is driven in this way is the synthesis of nucleic acids (polynucleotides) from nucleoside triphosphates, as illustrated on the right side of Figure 2–43.
Note that the repetitive condensation reactions that produce macromolecules can be oriented in one of two ways, giving rise to either the head polymerization or the tail polymerization of monomers. In so-called head polymerization, the reactive bond required for the condensation reaction is carried on the end of the
Figure 2–43 Synthesis of a polynucleotide, RNa or DNa, is a multistep process driven by aTP hydrolysis. in the first step, a nucleoside monophosphate is activated by the sequential transfer of the terminal phosphate groups from two aTp molecules. The high-energy intermediate formed—a nucleoside triphosphate—exists free in solution until it reacts with the growing end of an Rna or a Dna chain with release of pyrophosphate. hydrolysis of the latter to inorganic phosphate is highly favorable and helps to drive the overall reaction in the direction of polynucleotide synthesis. for details, see Chapter 5.