Amino Acid Metabolism

Fate of Dietary Protein Dietary protein Stomach: l, pepsin Denatured and partially hydrolyzed protein (large polypeptides) small intestine: proteases Amino acids and dipeptides intestinal lining: proteases Amino acids in bloodstream

verview of Amino Acid atabolism The catabolism of amino acids takes place in three stages: 1) Removal of the amino group, leaving the carbon skeleton of the amino acid. 3 N R 2) Breakdown of the carbon skeletons to a glycolytic intermediate, citric acid cycle intermediate, or acetyl-s-oa. small pieces 3) xidation of these intermediates to 2 and 2 with the production of ATP. 2 2 ATP

Representative Pathways of Amino Acid atabolism alanine N 4 + 3 3 N 3 pyruvate aspartate 2 N 3 N 4 + 2 oxaloacetate glutamine 2 N 2 2 N 3 2 N 4 + 2 2 α-ketoglutarate 2 N 3 2 N 4 + 3 2 + acetoacetate fumarate phenylalanine

pyruvate acetyl-oa acetoacetyl-oa alanine aspartate oxaloacetate citrate aconitate phenylalanine malate isocitrate fumarate oxalosuccinate succinate α-ketoglutarate phenylalanine succinyl-oa glutamine

pyruvate acetyl-oa acetoacetyl-oa ketones gluconeogenesis oxaloacetate citrate aconitate malate isocitrate fumarate oxalosuccinate succinate α-ketoglutarate succinyl-oa

Glucogenic and Ketogenic Amino Acids The α-keto acids derived from catabolism of many amino acids are intermediates in glycolysis and the citric acid cycle. These α-keto acids may, therefore, replenish citric acid cycle intermediates. Amino acids that can be converted into pyruvate, α-ketoglutarate, succinyl-oa, fumarate, and oxaloacetate can be converted into glucose by gluconeogenesis and are said to be glucogenic. The α-keto acids derived from catabolism of some amino acids are broken down to acetyl-oa or acetoacetyl-oa and are oxidized by the citric acid cycle or converted to ketone bodies. These amino acids are said to be ketogenic.

Glucogenic and Ketogenic Amino Acids alanine glycine threonine cysteine serine leucine isoleucine tryptophan threonine pyruvate acetyl-oa acetoacetyl-oa ketones aspartate asparagine gluconeogenesis oxaloacetate citrate aconitate leucine lysine phenylalanine tyrosine tryptophan malate isocitrate aspartate phenylalanine tyrosine fumarate oxalosuccinate succinate α-ketoglutarate isoleucine valine methionine threonine succinyl-oa glutamate glutamine arginine histidine proline

The Fate of the Amino Group of Amino Acids Nitrogen is present in the bloodstream in the form of ammonium ions: N4 + The normal concentration of N4 + ion in the blood is 3.0 x 10-5 to 6.0 x 10-5 M. Above these concentrations (hyperammonemia) coma may result. The extraction of the amino group from amino acids must be done in such a way so as not to increase blood ammonium ion levels above normal values.

The Fate of the Amino Group of Amino Acids The pathway for the extraction of amino groups from amino acids consists of three phases: 1) onversion of amino groups from all amino acids into a single product, glutamate, by transamination. 2) onversion of glutamate into α-ketoglutarate by oxidative deamination, releasing N4+. 3) onversion of N4+ into urea, which is extracted from the blood by the kidneys and excreted.

α-amino acid α-keto acid transamination α-keto glutarate glutamate oxidative deamination NAD, (NADP), N4+ 2 NAD+, (NADP)+, 2 aspartate 2 N 3 2 N Urea N 2 Urea cycle fumarate

Transamination

Transamination The first step in amino acid metabolism is the removal of the amino group by transamination followed by oxidative deamination. A transamination reaction can be represented as: N 3 + + N 3 R 1 R 2 R 1 R 2 Amino acid1 + α-ketoacid2 α-ketoacid1 + amino acid 2 Transamination reactions occur in all cells. The enzymes responsible for transaminations are called transaminases or amino transferases.

Transamination 3 N + + 3 N R 2 R 2 2 2 L-amino acid + α-ketoglutarate α-ketoacid + L-glutamate Most transaminases are specific for α-ketoglutarate but are less specific for the amino acid. This means that the amino groups of almost all amino acids end up on glutamic acid.

ne exception to this rule is in skeletal muscle, where transaminases use pyruvate as the amino acceptor, producing alanine as the product. 3 N + + 3 N R 3 R 3 L-amino acid + pyruvate α-ketoacid + L-alanine Another example of a specific transaminase reaction is aspartate transaminase: 3 N + + 3 N 2 2 2 2 2 2 L-aspartate + α-ketoglutarate oxaloacetate + L-glutamate

Transamination in Diagnostic Laboratory Medicine The presence of alanine transaminase (glutamate:pyruvate transaminase or GPT) and aspartate transaminase (glutamate:oxaloacetate transaminase, or GT) in the bloodstream, above a certain base level, may indicate liver damage. Serum GPT and GT (SGPT and SGT) tests measure the severity and stage of liver damage.

Vitamin B-6 as a oenzyme All transaminases require the coenzyme pyridoxal phosphate (derived from pyridoxine, vitamin B-6): 2 P 3 N pyridoxal phosphate Vitamin preparations may contain the precursor to pyridoxal phosphate in different forms: 2 2 2 N 2 3 2 N pyridoxine 2 3 N pyridoxamine 3 N pyridoxal

The Transaminase Mechanism R N 2 P 3 2- R N 2 P 3 2-3 N 3 N 2 2 3 N R 2 P 3 2-3 N 2 R P 2-3 3 N pyridoxal phosphate 3 N pyridoxamine phosphate

xidative Deamination glutamate 3 N 2 2 glutamate dehydrogenase 2 2 + N4+ α-ketoglutarate NAD+ (NADP+) 2 NAD (NADP) DES NT ENTER TE BLDSTREAM This reaction is reversible and provides a mechanism for 1) generating ammonium ion for excretion as urea 2) generating a-ketoglutarate 3) assimilating ammonium ion for use in other metabolic pathways in the liver and kidneys

Amino Group and Ammonia Transport Amino groups collected in extrahepatic tissues in the form of glutamate must be packaged in a non-toxic form for transport through the blood to the liver. Glutamate, itself, cannot pass through the cell membranes. Two different transport forms are used: 1) Production of glutamine in most cell types 2) Production of alanine in muscle cells.

Glutamine Production 3 N 2 2 glutamine synthetase 3 N + N4 + + ATP + + + ADP +Pi 2 2 N 2 L-glutamate L-glutamine In almost all cell types, glutamine synthetase catalyzes the formation of glutamine from glutamate: The glutamine, thus formed, is electrically neutral, nontoxic, and can pass through the cell membranes into the blood. The concentration of glutamine in the blood is higher than any other amino acid.

Amino Group and Ammonia Transport nce in the liver, the reverse reaction takes place and glutamine is deaminated to ammonium ion and glutamate. 3 N 2 3 N + 2 + N4 + 2 2 N 2 2 L-glutamine L-glutamate Glutamate can then be oxidatively deaminated to ammonium ion and α-ketoglutarate. 3 N 2 glutamate dehydrogenase 2 + NAD + + 2 + NAD + N4+ 2 2 L-glutamate α-ketoglutarate

In active muscle cells, large quantities of ammonium ion are produced. After two reactions, alanine is formed. Like glutamine, alanine is electrically neutral: 2 2 + NADP + N4+ glutamate dehydrogenase 3 N 2 2 + NADP + + 2 α-ketoglutarate L-glutamate 3 N 2 + + 3 N 3 2 3 2 pyruvate 2 alanine L-glutamate α-ketoglutarate

Like glutamine, alanine is electrically neutral and readily traverses membranes and enters the blood stream. 3 N 2 3 N 2 N 2 3 alanine L-glutamine In the liver, the combination of transamination and oxidative deamination reaction releases ammonium ions.

Glucose/Alanine ycle Liver Muscle tissue Glucose Glucose 6 ATP 2 ATP Urea Pyruvate Pyruvate Lactate 4 ATP N α-amino acid α-keto acid Alanine Alanine

Urea ycle verall Reaction: N4 + + 3 - + 3 ATP + aspartate (-N3 + ) urea + 2 ADP + AMP + 4 P4 3- + fumarate

Urea ycle ornithine arginine 3 N 3 N 2 2 2 2 2 2 N 2 N N 2 2 N N 2 N 3 urea 3 N 3 N 2 2 2 2 arginosuccinate 2 N 2 N N 2 2 N 2 N citrulline

Urea ycle glutamate urea ornithine glutamate a-ketoglutarate 2 ATP 3 - arginine N4 + 2 ADP,Pi fumarate Urea ycle ornithine citrulline carbamoyl phosphate Pi argininosuccinate AMP, 2Pi citrulline aspartate ATP cytosol mitochondria

Urea ycle ornithine N4 +, 3 -, 2ATP fumarate arginine 3 N 2 N 2 2 2 N N 2 2 N N 2 urea 3 N 2 2 2 N 3 2 N P 2ADP,Pi carbamoyl phosphate 3 N 2 3 N 2 P 2 2 arginosuccinate 2 N 2 N N 2 3 N 2 N 2 N citrulline 2 aspartate

Urea ycle verall Reaction: N4 + + 3 - + 3 ATP + aspartate urea + 2 ADP + AMP + 4 P4 3- + fumarate

Dietary protein Amino acid pool pyruvate, acetyl-oa, acetoacetate, TA cycle intermediates Ketones ATP, via TA cycle Fatty acids N4+ Glucose Urea Liver proteins Plasma proteins ther nitrogen-containing compounds

Nitrogen excretion products for various organisms -N2 groups aquatic invertebrates, bony fishes, crocodiles N3 mammals, sharks, some bony fishes, turtles 2 N birds, insects, reptiles, land gastropods N scorpions, spiders 2 N 2 N N ammonia urea uric acid guanine Water solubility Energy needed to produce N N N N N N

2 N 2 N 2 N 2 N N3 N3