for inter-tissue amino acid flux. Aspartate aminotransferase (AST) transfers the amino group of aspartate to 2-oxoglutarate, forming oxaloacetate and glutamate – however, this enzyme usually works in the reverse direction, its function being to convert glutamate (derived from amino acid funnelling, above) into aspartate, which is required to donate a second urea N-atom to the urea cycle. Since ALT and AST are both intracellular enzymes and widespread, necrosis of many tissues, including liver, causes them to increase in plasma; they are commonly used to diagnose hepatocellular damage (so-called ‘liver function tests’).
Figure 1.20 Transamination reactions. Transamination involves the transfer of an amino group from the α-carbon of an amino acid to a recipient 2-oxoacid (α-ketoacid), forming its corresponding 2-oxoacid and generating an amino acid. These reactions are catalysed by aminotransferase enzymes, all of which are readily reversible. Although no net deamination occurs, these reactions allow the amino groups of all amino acids to be ‘funnelled’ into key amino acids prior to net deamination and hence metabolism.
The second type of reaction responsible for amino acid deamination is oxidative deamination. Since most amino acids have been funnelled (deaminated) into glutamate by transamination, glutamate is the only amino acid that undergoes direct, oxidative, deamination, by glutamate dehydrogenase, regenerating 2-oxoglutarate and producing ammonia (NH3). Unusually for a highly regulated enzyme, glutamate dehydrogenase is reversible, and can use NAD+/NADH or NADP+/NADPH as electron carriers. In the ‘forward’ direction of deamination (catabolic, amino acid breakdown), it uses NAD+, but in the ‘reverse’ direction of amination of 2-oxoglutarate to glutamate (anabolic, amino acid synthesis) it uses NADPH, reflecting the different roles of these cofactors as redox energy carriers in different metabolic states (Box 1.8; for simplicity, ionisation states are not always shown as they would be at physiological pH; ammonium ion shown here [NH4+] may be considered the same as ammonia). Hence, aminotransferases (transamination) and glutamate dehydrogenase (oxidative deamination) work together to produce ammonia for detoxification to urea in the urea cycle, and carbon skeletons for further intermediary metabolism (Figure 1.21).
Figure 1.21 Deamination of amino acids. By linking transamination reactions to oxidative deamination, and then to the urea cycle, all amino acids can be efficiently deaminated, their carbon backbones (2-oxoacids) going on to further metabolism for energy production, and the amino group being safely detoxified by the urea cycle.
Box 1.8 Deamination
transamination is a type of deamination but does not remove net N
presence of α-amino group prevents oxidative breakdown
therefore α-amino group must be removed before catabolism can proceed
the nitrogen can be incorporated into other compounds or excreted in the urine
different types of deamination but oxidative deamination is quantitatively the most important
| • oxidative | glutamate dehydrogenase |
| • non-oxidative | serine & threonine: hydroxyl in side chain |
| • hydrolytic | asparagine & glutamine: N in side chain |
glutamate is the only amino acid that undergoes oxidative deamination (glutamate dehydrogenase)
mostly occurs in liver and kidney
unusually can use either NAD+ or NADP+ as coenzymeNAD+ used mostly in oxidative deaminationNADP+ used mostly in reductive amination
direction of reaction depends on substrate availability (& hence metabolic state)
allosteric regulation (unusually for a readily reversible reaction):
The urea (ornithine) cycle occurs in the liver. Urea (CO·(NH2)2) contains two nitrogen atoms: one derives from ammonia (oxidative deamination of glutamate), the other from aspartate (transamination, also of glutamate, by AST) (Figure 1.21): the body excretes nitrogen with minimal carbon (and energy) loss. Because urea is very water-soluble, much nitrogen waste can be excreted for relatively little water loss, an important adaptation in terrestrial animals. Urea lacks toxicity at physiological concentrations; it is (neuro)toxic only in extremely high concentrations, for example those seen in untreated renal failure, but considerably less so than ammonia.
The urea cycle starts by forming carbamoyl phosphate from ammonia by the enzyme carbamoyl phosphate synthase (Figure 1.22). This is the regulated step of the urea cycle, but since ammonia is so toxic, the entire urea cycle must have a high capacity in order to deal with any sudden influx of ammonia-nitrogen if amino acid deamination is acutely increased. This may occur for example in starvation, when endogenous protein is broken down to yield amino acids for deamination and gluconeogenesis, or following consumption of large amounts of protein in the diet (protein cannot be stored as such and therefore the excess amino acids ingested are converted to more efficient energy storage forms [lipid], again following deamination). The carbamoyl phosphate transfers the nitrogen to ornithine, one of a series of amino acid intermediates found in the urea cycle but not used in protein synthesis. The second nitrogen in the urea molecule is introduced by aspartate from aspartate transaminase (see above). Of note, the urea cycle spans both cytosolic and mitochondrial compartments of the liver cell, as well as being partially present in other tissues (intestine, kidney) – possibly a mechanism to ensure that ornithine is not limiting and always available to accept carbamoyl phosphate (and hence ammonia, which cannot be allowed to accumulate).
