Metabolic Fates of Amino Groups:- Pyridoxal Phosphate Participates in the Transfer of α-Amino Groups to α -Ketoglutarate
The first step in the catabolism of most L-amino acids, once they have reached the liver, is removal of the α-amino groups, promoted by enzymes called amino transferases or transaminases. In these transamination reactions, the α-amino group is transferred to the α-carbon atom of -ketoglutarate, leaving behind the corresponding -keto acid analog of the amino acid (Fig. 18–4). There is no net deamination (loss of amino groups) in these reactions, because the α-ketoglutarate becomes aminated as the α-amino acid is deaminated. The effect of transamination reactions is to collect the amino groups from many different amino acids in the form of L-glutamate. The glutamate then functions as an amino group donor for biosynthetic pathways or for excretion pathways that lead to the elimination of nitrogenous waste products. Cells contain different types of aminotransferases. Many are specific for -ketoglutarate as the amino group acceptor but differ in their specificity for the L-amino acid. The enzymes are named for the amino group donor (alanine aminotransferase, aspartate aminotransferase, for example). The reactions catalyzed by aminotransferases are freely reversible, having an equilibrium con stant of about 1.0 (ΔG ≈0 kJ/mol).
FIGURE 18–4 Enzyme-catalyzed transaminations. In many amino transferase reactions, -ketoglutarate is the amino group acceptor. All aminotransferases have pyridoxal phosphate (PLP) as cofactor. Al though the reaction is shown here in the direction of transfer of the amino group to α-ketoglutarate, it is readily reversible.
All aminotransferases have the same prosthetic group and the same reaction mechanism. The prosthetic group is pyridoxal phosphate (PLP), the coenzyme form of pyridoxine, or vitamin B6. We encountered pyridoxal phosphate in Chapter 15, as a coenzyme in the glycogen phosphorylase reaction, but its role in that reaction is not representative of its usual coenzyme function. Its primary role in cells is in the metabolism of molecules with amino groups. Pyridoxal phosphate functions as an intermediate carrier of amino groups at the active site of amino transferases. It undergoes reversible transformations between its aldehyde form, pyridoxal phosphate, which can accept an amino group, and its aminated form, pyridoxamine phosphate, which can donate its amino group to an -keto acid (Fig. 18–5a). Pyridoxal phosphate is generally covalently bound to the enzyme’s active site through an aldimine (Schiff base) linkage to the -amino group of a Lys residue (Fig. 18–5b, d). Pyridoxal phosphate participates in a variety of re actions at the α, β, γ and carbons (C-2 to C-4) of amino acids. Reactions at the α carbon (Fig. 18–6) include racemizations (interconverting L- and D-amino acids) and decarboxylations, as well as transaminations. Pyridoxal phosphate plays the same chemical role in each of these reactions. A bond to the α carbon of the substrate is broken, removing either a proton or a carboxyl group. The electron pair left behind on the carbon would form a highly unstable carbanion, but pyridoxal phosphate provides resonance stabilization of this intermediate (Fig. 18–6 inset). The highly conjugated structure of PLP (an electron sink) permits delocalization of the negative charge. Aminotransferases (Fig. 18–5) are classic examples of enzymes catalyzing bimolecular Ping-Pong reactions (see Fig. 6–13b), in which the first substrate reacts and the product must leave the active site before the second substrate can bind. Thus, the incoming amino acid binds to the active site, donates its amino group to pyridoxal phosphate, and departs in the form of an α-keto acid. The incoming α-keto acid then binds, accepts the amino group from pyridoxamine phosphate, and departs in the form of an amino acid. As described in Box 18–1 on page 664, measurement of the alanine aminotransferase and aspartate aminotransferase levels in blood serum is important in some medical diagnoses.
FIGURE 18–5 Pyridoxal phosphate, the prosthetic group of amino transferases. (a) Pyridoxal phosphate (PLP) and its aminated form, pyridoxamine phosphate, are the tightly bound coenzymes of amino transferases. The functional groups are shaded. (b) Pyridoxal phosphate is bound to the enzyme through noncovalent interactions and a Schiff base linkage to a Lys residue at the active site. The steps in the formation of a Schiff base from a primary amine and a carbonyl group are detailed in Figure 14–5. (c) PLP (red) bound to one of the two active sites of the dimeric enzyme aspartate aminotransferase, a typical aminotransferase; (d) close-up view of the active site, with PLP (red, with yellow phosphorus) in aldimine linkage with the side chain of Lys258 (purple); (e) another close-up view of the active site, with PLP linked to the substrate analog 2-methylaspartate (green) via a Schiff base (PDB ID 1AJS).
