Overview of Nitrogen Metabolism:- Several Classes of Reactions Play Special Roles in the Biosynthesis of Amino Acids and Nucleotides
The pathways described in this chapter include a variety of interesting chemical rearrangements. Several of these recur and deserve special note before we progress to the pathways themselves. These are (1) transamination reactions and other rearrangements promoted by enzymes containing pyridoxal phosphate; (2) transfer of one-carbon groups, with either tetrahydrofolate (usually at the -CH- and -CH2OH oxidation levels) or S adenosylmethionine (at the -CH3 oxidation level) as cofactor; and (3) transfer of amino groups derived from the amide nitrogen of glutamine. Pyridoxal phosphate (PLP), tetrahydrofolate (H4 folate), and S-adenosylme thionine (adoMet) were described in some detail in Chapter 18 (see Figs 18–6, 18–17, and 18–18). Here we focus on amino group transfer involving the amide nitrogen of glutamine. More than a dozen known biosynthetic reactions use glutamine as the major physiological source of amino groups, and most of these occur in the pathways out lined in this chapter. As a class, the enzymes catalyzing these reactions are called glutamine amidotransferases. All have two structural domains: one binding glutamine, the other binding the second substrate, which serves as amino group acceptor (Fig. 1). A conserved Cys residue in the glutamine-binding domain is believed to act as a nucleophile, cleaving the amide bond of glutamine and forming a covalent glutamyl-enzyme inter mediate. The NH3 produced in this reaction is not released, but instead is transferred through an “ammonia channel” to a second active site, where it reacts with the second substrate to form the aminated product. The covalent intermediate is hydrolyzed to the free enzyme and glutamate. If the second substrate must be activated, the usual method is the use of ATP to generate an acyl phosphate intermediate (ROOX in Fig. 1, with X as a phosphoryl group). The enzyme glutaminase acts in a similar fashion but uses H2O as the second substrate, yielding NH+4 and glutamate (see Fig. 18–8).
MECHANISM FIGURE 1 Proposed mechanism for glutamine amidotransferases. Each enzyme has two domains. The glutamine binding domain contains structural elements conserved among many of these enzymes, including a Cys residue required for activity. The NH3-acceptor (second-substrate) domain varies. 1 The γ-amido nitrogen of glutamine (red) is released as NH3 in a reaction that prob ably involves a covalent glutamyl-enzyme intermediate. The NH3 travels through a channel to the second active site, where 2 it reacts with any of several acceptors. Two types of amino acceptors are shown. X represents an activating group, typically a phosphoryl group derived from ATP, that facilitates displacement of a hydroxyl group from ROOH by NH3.
