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1、Amino Acid Oxidation and the Production of UreaAmino acids can be used as fuels in animals (especially in carnivores)Dietary proteins are enzymatically degraded to amino acidsEnzymatic digestion begins in stomach and ends in small intestinenPepsin cleaves polypeptides into smaller peptides in stomac
2、h (N-term of Y, F, W);nTrypsin and chymotrypsin further cleave the peptides in small intestine;nCarboxypeptidase and aminopeptidase cleave the small peptides into amino acids, which are then absorbed and eventually delivered to liver.Amino acids in excess can neither be stored, nor excreted, but are
3、 oxidized as fuelsAmino groups are separated from the carbon skeletons at the beginningnThe amino group is reused or excreted (as ammonia, urea or uric acid);nThe carbon skeletons (a-keto acids) generally find their way to the citric acid cycle for further oxidation or conversion.Most of the amino a
4、cids are metabolized in the liver: the amino groups are delivered from extrahepatic tissues to liver via Glu or Glna-ketoglutarate collects the a-amino groups from many amino acidsnAminotransferases (transaminases) catalyze these transamination reactions to form Glu and a-keto acids;nAll use pyridox
5、al phosphate (PLP，吡哆醛磷酸) as the prosthetic group;nReactions are freely reversible.PLP covalently binds to the enzyme and transiently carries an amino groupnPLP is derived from pyridoxine (vitamin B6);nPLP binds to the enzyme via covalent and noncovalent bonds;nThe aldehyde form and aminated form(pyr
6、idoxamine) of the prosthetic group can easily interchange, taking or releasing an amino group.The transamination reaction via PLP takes a ping-pong mechanismnOne amino acid transfers its amino group to PLP and then departs the active site as an a-keto acid;nAnother a-keto acid moves in and takes the
7、 amino group from pyridoxamine phosphate and leaving as an amino acid (Glu).Glutamate collects and delivers ammonia to the liver nGlutamine synthetase catalyzes the addition of ammonia to Glu to yield Gln;nATP is used to activate the -COOH (forming g -glutamyl phosphate) for amination;nAmmonia is to
8、xic to animal tissues, especially the brain (pH, depletion of ATP and neurotransmitters).Pyruvate collects amino groups (converting to alanine) from muscle and deliver them to the liverBoth pyruvate and ammonia are moved from muscle to liver via alaninenAlanine transfers its amino group to a- ketogl
9、utarate or oxaloacetate in liver;nPyruvate is reconverted to glucose in liver;nThis alanine-glucose cycle solves two problems at the same time for vigorously contracting skeletal muscles.The amino groups of Gln and Glu can be removed as ammonia in liver mitochondriaGlutaminase catalyzes the release
10、of NH4+ from GlnnThe released NH4+ can be directly excreted in most aquatic vertebrates, thus called ammonotelic animals (排氨 动物）;nOr converted to urea in many terrestrial animals (ureotelic animals).Glutamate dehydrogenase releases NH4+ from GlunEither NAD+ or NADP+ can be the cofactors;nThe enzyme
11、is inhibited by GTP and activated by ADP;nGTP and ADP signals the requirements of a-ketoglutarate for the TCA cycle.In ureotelic animals, the ammonia in the mitochondria of hepatocytes is converted to urea via a cycling pathway: the Urea Cycle (the first cycling pathway ever discovered!) The urea cy
12、cle spans the mitochondria and cytosolCarbamoyl phosphate is formed in the mitochondri al matrixnCatalyzed by carbamoyl phosphate synthetase I;nAmmonia ion is added to an phosphorylated (activated) bicarbonatenThe carbamate is activated by another phosphorylation;nThe enzyme is activated by N- acety
13、lglutamate.The carbamoyl group adds to ornithine to form citrullinenCatalyzed by ornithine transcarbamoylase;nThis occurs in the mitochondria;nCitrulline is released into the cytosol.An Asp adds to citrulline to form argininosuccinatenCatalyzed by argininosuccinate synthetase in cytosol;nAn citrully
14、l-AMP intermediate is formednAspartate aminotransferase catalyzes the transferring of the amino group from Glu to oxaloacetate, forming Asp in mitochondria matrix;n(the red part is the cofactor, what is it?)The amino group of Asp could also be obtained from Glu Argininosuccinate is cleave to form ar
15、ginine and fumaratenCatalyzed by argininosuccinate lyase;nAmino group of Asp (also from Glu) is used to form Arg;nThe carbon skeleton of Asp is converted to fumarate, which enters the citric acid cycle. Arginine is cleave to form urea and ornithinenCatalyzed by arginase;nornithine is regenerated;nOr
16、nithine enters mitochondria to start another round of urea cycle.The urea cycle and citric acid cycle are linked via fumaratenFumarate is converted to oxaloacetate by the action of fumarase and malate dehydrogenase;nOxaloacetate is converted to Asp by transamination reaction;nAsp reenters the urea cycle.The rate of urea synthesis is controllednFour high-energy phosphat
