Nh

Schiff base oh ch3

FIGURE 18-5 Pyridoxal phosphate, the prosthetic group of aminotransferases. (a) Pyridoxal phosphate (PLP) and its aminated form, pyri-doxamine phosphate, are the tightly bound coenzymes of aminotransferases. The functional groups are shaded. (b) Pyridoxal phosphate is bound to the enzyme through noncovalent interactions and a Schiffbase 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).

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).

Glutamate Releases Its Amino Group as Ammonia in the Liver

As we have seen, the amino groups from many of the a-amino acids are collected in the liver in the form of the amino group of l-glutamate molecules. These amino groups must next be removed from glutamate to prepare them for excretion. In hepatocytes, glutamate is transported from the cytosol into mitochondria, where it undergoes oxidative deamination catalyzed by l-glutamate dehydrogenase (Mr 330,000). In mammals, this enzyme is present in the mitochondrial matrix. It is the only enzyme that can use either NAD+ or NADP+ as the acceptor of reducing equivalents (Fig. 18-7).

The combined action of an aminotransferase and glutamate dehydrogenase is referred to as transdeam-ination. A few amino acids bypass the transdeamina-

tion pathway and undergo direct oxidative deamination. The fate of the NH4 produced by any of these deamination processes is discussed in detail in Section 18.2. The a-ketoglutarate formed from glutamate deamina-tion can be used in the citric acid cycle and for glucose synthesis.

Glutamate dehydrogenase operates at an important intersection of carbon and nitrogen metabolism. An allosteric enzyme with six identical subunits, its activity is influenced by a complicated array of allosteric modulators. The best-studied of these are the positive modulator ADP and the negative modulator GTP. The metabolic rationale for this regulatory pattern has not been elucidated in detail. Mutations that alter the allosteric binding site for GTP or otherwise cause permanent activation of glutamate dehydrogenase lead to a human genetic disorder called hyperinsulinism-hyperammonemia

NH3 Amine

Pyridoxal phosphate (aldimine form, on regenerated enzyme)

NH3 Amine

Pyridoxal phosphate (aldimine form, on regenerated enzyme)

MECHANISM FIGURE 18-6 Some amino acid transformations at the a carbon that are facilitated by pyridoxal phosphate. Pyridoxal phosphate is generally bonded to the enzyme through a Schiff base (see Fig. 18—5b, d). Reactions begin (top left) with formation of a new Schiff base (aldimine) between the a-amino group of the amino acid and PLP, which substitutes for the enzyme-PLP linkage. Three alternative fates for this Schiff base are shown: @ transamination, @ racemiza-tion, and © decarboxylation. The Schiff base formed between PLP and the amino acid is in conjugation with the pyridine ring, an electron sink that permits delocalization of an electron pair to avoid formation of an unstable carbanion on the a carbon (inset). A quinonoid intermediate is involved in all three types of reactions. The transamination route (®) is especially important in the pathways described in this chapter. The pathway highlighted here (shown left to right) represents only part of the overall reaction catalyzed by aminotransferases. To complete the process, a second a-keto acid replaces the one that is released, and this is converted to an amino acid in a reversal of the reaction steps (right to left). Pyridoxal phosphate is also involved in certain reactions at the fi and y carbons of some amino acids (not shown). ^ Pyridoxal Phosphate Reaction Mechanisms syndrome, characterized by elevated levels of ammonia in the bloodstream and hypoglycemia.

Glutamine Transports Ammonia in the Bloodstream

Ammonia is quite toxic to animal tissues (we examine some possible reasons for this toxicity later), and the levels present in blood are regulated. In many tissues, including the brain, some processes such as nucleotide degradation generate free ammonia. In most animals much of the free ammonia is converted to a nontoxic compound before export from the extrahepatic tissues into the blood and transport to the liver or kidneys. For this transport function, glutamate, critical to intracellular amino group metabolism, is supplanted by l-glutamine. The free ammonia produced in tissues is combined with glutamate to yield glutamine by the action of glutamine synthetase. This reaction requires

COO"

Quick Permanent Weight Loss

Quick Permanent Weight Loss

A Step By Step Guide To Fast Fat Loss. Do you ever feel like getting rid of the extra weight of your body? If you do, it‟s quite normal because

Get My Free Ebook


Post a comment