Amino acid metabolism The important reaction commonly employed in the breakdown of an amino acid is always the removal of its -amino group. The product ammonia is excreted after conversion to urea or other products and the carbon skeleton is degraded to CO2 releasing energy. Metabolism of amino acids differs, but 3 common reactions: Transamination Deamination Decarboxylation
In transamination Transamination Amino acids are degraded in the liver. An amino group is transferred from an amino acid to an -keto acid, usually -ketoglutarate. The reaction is catalyzed by a transaminase or aminotransferase. A new amino acid, usually glutamate, and a new -keto acid are formed. 2
Transamination reactions
Enzymatic Transamination Typically, -ketoglutarate accepts amino groups L-Glutamine acts as a temporary storage of nitrogen L-Glutamine can donate the amino group when needed for amino acid biosynthesis All aminotransferases rely on the pyridoxal phosphate cofactor
Amino Group Transfer - Aminotransferase Enzymatic removal of -amino groups (transaminase /aminotransferases - named for amino donor i.e. Ala aminotranferase removes amino group from Ala)
Ping-pong kinetics of aspartate transaminase (next slide)
(from previous slide)
Transamination
COO COO CH 2 CH 2 CH 3 CH 2 CH 3 CH 2 HC NH 3 + + C O C O + HC NH 3 + COO COO COO COO alanine -ketoglutarate pyruvate glutamate Aminotransferase (Transaminase) The 3-C -keto acid pyruvate is produced from alanine, cysteine, glycine, serine, & threonine. Alanine deamination via Transaminase directly yields pyruvate.
COO COO COO CH 2 COO CH 2 CH 2 CH 2 CH 2 CH 2 HC COO NH 3 + + C O C O + COO COO HC COO NH 3 + aspartate -ketoglutarate oxaloacetate glutamate Aminotransferase (Transaminase) The 4-C Krebs Cycle intermediate oxaloacetate is produced from aspartate & asparagine. Aspartate transamination yields oxaloacetate. Aspartate is also converted to fumarate in Urea Cycle. Fumarate is converted to oxaloacetate in Krebs cycle.
The Amino Group is Removed From All Amino Acids First
Oxidative Deamination Oxidative deamination Removes the amino group as an ammonium ion from glutamate. Provides -ketoglutarate for transamination. 12
Oxidative Deamination Glutamate formed by transamination reactions is deaminated to -ketoglutarate Glutamate dehydrogenase - NAD + or NADP + is coenzyme Other AA oxidases - (liver, kidney) low activity
Glutamate Dehydrogenase catalyzes a major reaction that effects net removal of N from the amino OOC glutamate OOC C C C COO -ketoglutarate acid pool. It is one of the few enzymes that can use NAD + or NADP + as e acceptor. Oxidation at the -carbon is followed by hydrolysis, releasing NH 4+. H 2 C H 2 H 2 O + NH 3 H 2 C C COO H 2 H O NAD(P) + NAD(P)H + NH 4 + Glutamate Dehydrogenase
Amino acid -ketoglutarate NADH + NH 4 + -keto acid glutamate NAD + + H 2 O Transaminase Glutamate Dehydrogenase Summarized above: The role of transaminases in funneling amino N to glutamate, which is deaminated via Glutamate Dehydrogenase, producing NH 4+.
Non-oxidative deamination Amino acids such as serine and histidine are deaminated non-oxidatively The other reactions involved in the catabolism of amino acids are decarboxylation, transulfuration, desulfuration, dehydration etc.
DEAMIDATION The amino acid, which contains an amide linkage with ammonia at the γ-carboxyl, degraded by process of deamidation. E.g, conversion of asparagine to aspartate by removal of alpha amino group.
Decarboxylation The decarboxylation process is important since the products of decarboxylation reactions give rise to physiologically active amines. The enzymes, amino acid decarboxylases are pyridoxal phosphate dependent enzymes. Pyridoxal phosphate forms a Schiff's base with the amino acid so as to stabilise the -carbanion formed by the cleavage of bond between carboxyl and -carbon atom. The physiologically active amines epinephrine, nor-epinephrine, dopamine, serotonin, -amino butyrate and histamine are formed through decarboxylation of the corresponding precursor amino acids.
Transmethylation Resynthesis of methionine: Homocysteine accepts a methyl group from N5- methyltetrahydrofolate (N5-methyl-THF) requiring methylcobalamin, a coenzyme derived from vitamin B12. The methyl group is transferred from the B12 derivative to homocysteine, and cobalamin is recharged from N5-methyl-THF.
Excretory Forms of Nitrogen
Fate of Individual Amino Acids Seven to acetyl-coa Leu, Ile, Thr, Lys, Phe, Tyr, Trp Six to pyruvate Ala, Cys, Gly, Ser, Thr, Trp Five to -ketoglutarate Arg, Glu, Gln, His, Pro Four to succinyl-coa Ile, Met, Thr, Val Two to fumarate Phe, Tyr Two to oxaloacetate Asp, Asn
Summar y of Amino Acid Cataboli sm
NITROGEN BALANCE Nitrogen balance = nitrogen ingested - nitrogen excreted (primarily as protein) (primarily as urea) Nitrogen balance = 0 (nitrogen equilibrium) protein synthesis = protein degradation Positive nitrogen balance protein synthesis > protein degradation Negative nitrogen balance protein synthesis < protein degradation
CLE ation nia ()
REGULATION OF UREA CYCLE: N-Acetylglutamate is an essential activator for carbamoyl phosphate synthetase I the rate-limiting step in the urea cycle N-Acetylglutamate is synthesized from acetyl coenzyme A and glutamate by N-acetylglutamate synthase in a reaction for which arginine is an activator.
UREMIA urea and other waste products, are retained in the blood. Early symptoms include anorexia, lethargy,fatigue, nausea, vomiting, cold, bone pain, itch, shortness of breath,, and late symptoms can include decreased mental acuity and coma. when the glomerular filtration rate, a measure of kidney function, is below 50% of normal. [2] Uremia can also result in uremic pericarditis. There are many dysfunctions caused by uremia affecting many systems of the body, such as blood (lower levels of erythropoietin), sex (lower levels of testosterone/estrogen), and bones (osteoporosis and metastatic calcifications). Uremia can also cause decreased peripheral conversion of T4 to T3, producing a functionally hypothyroid state.
Azotemia: Refers to high levels of urea, but is used primarily when the abnormality can be measured chemically but is not yet so severe as to produce symptoms. Uremia is the pathological manifestations of severe azotemia.
Hyperammonemia The capacity of the hepatic urea cycle exceeds the normal rates of ammonia generation. The levels of serum ammonia are normally low (5 35 μmol/l). CAUSES: liver function is compromised, due to genetic defects of the urea cycle or liver disease. blood levels can rise above 1,000 μmol/l. Such hy per ammon emia is a medical emergency, because ammonia has a direct neurotoxi effect on the CNS.