Amino acid pool is the entire collection of free amino acids in the body.

Turnover is the continual renewal or replacement of biomolecules; for protein it is defined by the balance between protein synthesis and protein degradation.

General Process for Amino Acid Catabolism:

• Removal of the amino group

• Use of the removed ¬NH2 in the synthesis of new nitrogen compounds

• Passage of nitrogen into the urea cycle

• Incorporation of the carbon atoms into compounds that can enter the citric acid cycle

Human bodies do not store nitrogen-containing compounds, and ammonia is toxic to cells. Therefore, the amino nitrogen from dietary protein has just two possible fates:

• It must either be incorporated into urea and excreted

• Be used in the synthesis of new nitrogen-containing compounds; these include the following:

  • Nitric oxide (NO, a chemical messenger)

  • Hormones

  • Neurotransmitters

  • Nicotinamide (in coenzymes NAD+ and NADP+)

  • Heme (as part of hemoglobin in red blood cells)

  • Purine and pyrimidine bases (for nucleic acids)

Transamination is the interchange of the amino group of an amino acid and the keto group of an alpha- keto acid.

• A number of transaminase enzymes are responsible for “transporting” an amino group from one molecule to another.

  • Most are specific for alpha-ketoglutarate as the amino-group acceptor and can remove the ¬ NH2 group (deaminate) from several different amino acids.

  • The alpha-ketoglutarate is converted to glutamate, and the amino acid is converted to an alpha-keto acid.

• The enzyme for this conversion, alanine aminotransferase (ALT), is especially abundant in the liver, and above-normal ALT concentrations in the blood are taken as an indication of liver damage that has allowed ALT to leak into the bloodstream.

Oxidative deamination is the conversion of an amino acid ¬NH2 group to an alpha-keto group, with removal of NH4 +.

Direct excretion of ammonia in urine is not feasible for mammals, because the volume of water needed to accomplish this safely would cause dehydration. Mammals must first convert ammonia, in solution as ammonium ion, to nontoxic urea via the urea cycle.

• STEPS 1 AND 2 OF THE UREA CYCLE: Building Up a Reactive Intermediate:

  • The first step of the urea cycle transfers the carbamoyl group, H2NC=O, from carbamoyl phosphate to ornithine, an amino acid not found in proteins, to give citrulline, another non-protein amino acid.

  • This exergonic reaction introduces the first urea nitrogen into the urea cycle. In Step 2, a molecule of aspartate combines with citrulline in a reaction driven by conversion of ATP to adenosine monophosphate (AMP) and pyrophosphate (P2O7 4-), followed by the additional exergonic hydrolysis of pyrophosphate.

  • Both nitrogen atoms destined for elimination as urea are now bonded to the same carbon atom in arginino-succinate.

• STEPS 3 AND 4 OF THE UREA CYCLE: Cleavage and Hydrolysis of the Step 2 Product Step 3 cleaves arginino-succinate into two pieces: arginine, an amino acid, and fumarate.

• In step 4, hydrolysis of arginine to give urea and regenerate the reactant in step 1 of the cycle, ornithine takes place.

Those amino acids that are converted to acetoacetyl-CoA or acetyl-CoA then enter the ketogenesis pathway and are called ketogenic amino acids.

Those amino acids that proceed by way of oxaloacetate to the gluconeogenesis pathway are known as glucogenic amino acids. Both ketogenic and glucogenic amino acids are able to enter fatty acid biosynthesis via acetyl-CoA.

Nonessential amino acid: one of 11 amino acids that are synthesized in the body and are therefore not necessary in the diet.

Essential amino acid : an amino acid that cannot be synthesized by the body and thus must be obtained from the diet.

Reductive amination : conversion of an alpha-keto acid to an amino acid by reaction with NH4 +.