Dynamics of Protein And Amino Acid MetabolismDietary Proteins Digestion to Amino Acids Transport in Blood to Cells Protein Synthesis Functional Proteins Protein Degradation In Amino Acids Proteasomes Following Tagging With Ubiquitin Metabolites 2
Intestinal Absorption Amino Acids OligopeptidesLumen Transport Protein Oligopeptides Peptidases Amino Acids Blood 3
AMINO ACID METABOLISM BODY PROTEINS Proteosynthesis 250 – 300 Degradation g/day NONPROTEIN AMINO ACIDSDIETARY DERIVATIVESPROTEINS (Carbon skeleton) Porphyrins Purines Conversion PyrimidinesGLYCOLYSIS NeurotransmittersKREBS CYCLE Hormones UREA NH3 Komplex lipids Aminosugars GLUCOSE ACETYL CoA CO2 KETONBODIES
ENZYMES CLEAVING THE PEPTIDE CHAINEndopeptidases – hydrolyse the peptide bond inside a chain Pepsin, trypsin, chymotrypsinExopeptidases – split the peptide bond at the end of a protein molecule Aminopeptidase, carboxypeptidases Dipeptidases Pepsin (pH 1.5 – 2.5) – peptide bond derived fromTyr, Phe, bonds between Leu and Glu Trypsin (pH 7.5 – 8.5) – bonds between Lys a Arg Chymotrypsin (pH 7.5 – 8.5) – bonds between Phe a Tyr
Essential Amino Acids in HumansRequired in dietHumans incapable of forming requisite carbon skeleton Arginine* Lysine Histidine* Methionine Isoleucine Threonine Leucine Phenylalanine Valine Tryptophan *Required to some degree in young growing period and/or sometimes during illness.
Non-essential and nonessential amino acids in humansNot required in dietCan be formed from a-keto acids by transamination and subsequent reactions Alanine Glycine Asparagine Proline Aspartate Serine Glutamate Cysteine (from Met*) Glutamine Tyrosine (from Phe*) * Essential amino acids
Ketogenic Amino AcidsYield acetyl CoA or acetoacetate Lysine Leucine
General reactions of amino acids aretransamination and deamination of a-amino group Oxidative deamination a-keto acid Transaminatoin a-keto acid NH2 + amino acid R CH COOH Oxidative amin decarboxylationTransamination – the transfer of the amino group to a suitable keto acid acceptor.Oxidative deamination - the amino acid is converted into the corresponding keto acid by theremoval of the amine functional group as ammonia and the amine functional group is replaced by theketone group. The ammonia eventually goes into the urea cycle.Oxidative decarboxylation – the formation of biogenic amines.
Transamination reactionThe first step in the catabolism of most amino acids is removal of a-amino groups by enzymes aminotransferases or transaminasesAll aminotransferases have the same prostethic group and the same reaction mechanism. The prostethic group is pyridoxal phosphate (PPL), the coenzyme form of pyridoxine (vitamin B6)
Mechanism of transamination reaction: PPL complex with enzyme accept an amino group to formpyridoxamine phosphate, which can donate its amio group to an a-keto acid (Aldimine) (Ketimine) Pyridoxal phosphate Schiff base Pyridoxamine phosphate (Aldimin) (Ketimin)
Aminotransferases are differ in their specificity for L- amino acids. The enzymes are named for the amino group donor Clinicaly important transaminases Alanine-a-ketoglutarate transferase ALT (also called glutamate-pyruvate transaminase – GPT) Aspartate-a-ketoglutarate transferase AST (also called glutamate-oxalacetate transferase – GOT)Important in the diagnosis of heart and liver damage caused by heart attack, drug toxicity, or infection.
Glucose-alanine cycle Alanine plays a special role in transporting amino groups to liver. Ala is the carrier of ammonia and of the carbon skeleton of pyruvate from muscle to liver. The ammonia is excreted and the pyruvate is used to produce glucose, which is returned to the muscle.According to D. L. Nelson, M. M. Cox :LEHNINGER. PRINCIPLES OF BIOCHEMISTRY Fifth edition
Glutamate releases its amino group as ammonia in the liver 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.• Glutamate undergoes oxidative deamination catalyzed by L-glutamate dehydrogenase.• Enzyme is present in mitochondrial matrix.• It is the only enzyme that can use either NAD+ or NADP+ as the acceptor of reducing equivalents.• Combine action of an aminotransferase and glutamate dehydrogenase referred to as transdeamination.
Ammonia transport in the form of glutamineExcess ammonia is added toglutamate to form glutamine. Glutamine synthetase Glutamine enters the liver and NH4+ is liberated in mitochondria by the enzyme glutaminase. Ammonia is remove by urea synthesis.
A. Oxidative deamination Oxidative deamination •L-amino acid oxidase produces Amino acids + FMN + H2O ammonia and a-keto acid directly, using FMN as cofactor. L-amino acid oxidase •The reduced form of flavin must be regenerated by O2 molecule. a-keto acids + FMNH2 + NH3 •This reaction produces H2O2 molecule which is decompensated by catalase. O2 catalse H2O + O2 FMN H2O2B. Nonoxidative deamination Is possible only for hydroxy amino acids serine threonine Serin-threonin dehydratase pyruvate + NH3 a-ketoglutate + NH3
Amino acid metabolism and central metabolic pathways 20 amino acids are converted to 7 products: pyruvate acetyl-CoA acetoacetate a-ketoglutarate succynyl-CoA oxalacetate fumarate
Both glucogenic and ketogenic amino acids Yield a-ketoglutarate, pyruvate, oxaloacetate, fumarate, or succinyl-CoA in addition to acetyl CoA or acetoacetate Isoleucine Phenylalanine Threonine Tyrosine Tryptophan
Glycine biosynthesisGlycine produced from serine or from the diet can also be oxidized by glycinedecarboxylase (also referred to as the glycine cleavage complex, GCC) to yield a secondequivalent of N5,N10-methylene-tetrahydrofolate as well as ammonia and CO2.
Serine biosynthesis from glycineReaction involves the transfer of the hydroxymethyl group from serine to the cofactortetrahydrofolate (THF), producing glycine and N5,N10-methylene-THF.
Cysteine and methionine biosynthesisThe sulfur for cysteine synthesis comes from the essential amino acid methionine. SAM Condensation of ATP and methionine yield S-adenosylmethionine (SAM)SAM serves as a precurosor for numerous methyl transfer reactions (e.g. the conversionof norepinephrine to epinenephrine).
Utilization of methionine in the synthesis of cysteine * Conversion of homocysteine back to Met. N5- methyl-THF is donor of methyl group. *folate + vit B121. Conversion of SAM to homocysteine.2. Condensation of homocysteine with serine to cystathione.3. Cystathione is cleavaged to cysteine.
Homocystinuria Genetic defects for both the synthase and the lyase. Missing or impaired cystathionine synthase leads to homocystinuria. High concentration of homocysteine and methionine in the urine. Homocysteine is highly reactive molecule.Disease is often associated with mental retardation, multisystemic disorder of connective tissue, muscle, CNS, and cardiovascular system.
Biosynthesis of Tyrosine from PhenylalaninePhenylalanine hydroxylase is a mixed-function oxygenase: one atom of oxygen is incorporated intowater and the other into the hydroxyl of tyrosine. The reductant is the tetrahydrofolate-related cofactortetrahydrobiopterin, which is maintained in the reduced state by the NADH-dependent enzymedihydropteridine reductase
Phenylketonuria Missing or deficient phenylalanine hydroxylase results in hyperphenylalaninemia. Phenylketonuria is the most widely recognized hyperphenylalaninemia (and most severe) It is the genetic disease. The mental retardation is caused by the accumulation ofphenylalanine, which becomes a major donor of amino groups in aminotransferase activity and depletes neural tissue of α- ketoglutarate. Absence of α-ketoglutarate in the brain shuts down the TCAcycle and the associated production of aerobic energy, which is essential to normal brain development.
Enzymes which metabolised amino acides containe vitamines as cofactors Vater soluble vitamins B THIAMINE B1 (thiamine diphosphate) oxidative decarboxylation of a-ketoacidsRIBOFLAVIN B2 (flavin mononucleotide FMN, flavin adenine dinucleotide FAD) oxidses of a-aminoacids NIACIN B3 – nicotinic acid (nikotinamide adenine dinucleotide NAD+ nikotinamide adenine dinukleotide phosphate NADP+) dehydrogenases, reductase PYRIDOXIN B6 (pyridoxalphosphate) transamination reaction and decarboxylation FOLIC ACID (tetrahydropholate) Meny enzymes of amino acid metabolism
Nitrogenous derivatives of amino acids Glycine heme, purine, creatine, conjugation of bile acids Histidine histamine Ornithine a arginin creatine, polyamines (spermidine, spermine) Tryptophan serotonine (melatonine) Tyrosine Epinephrine, norepinephrine Glutamic acid g-aminobutyric acid (GABA)