chapters 18 - amino acd oxidation , production of urea biochemistry
TRANSCRIPT
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18| Amino Acid Oxidation Production of Urea
© 2013 W. H. Freeman and Company
22| Nitrogen Assimilation, Biosynthetic Use, and Excretion
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The use of amino acids as fuel varies greatly by organism
• About 90% of energy needs of carnivores can be met by amino acids immediately after a meal
• Microorganisms scavenge amino acids from their environment for fuel when needed
• Only a small fraction of energy needs of herbivores are met by amino acids
• Plants do not use amino acids as a fuel source, but can degrade amino acids to form other metabolites
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Metabolic Circumstances of Amino Acid Oxidation
• Leftover amino acids from normal protein turnover
• Dietary amino acids that exceed body’s protein synthesis needs
• Proteins in the body can be broken down to supply amino acids for energy when carbohydrates are scarce (starvation, diabetes mellitus)
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Dietary proteins are enzymatically hydrolyzed into amino acids
• Pepsin cuts protein into peptides in the stomach
• Trypsin and chymotrypsin cut proteins and larger peptides into smaller peptides in the small intestine
• Aminopeptidase and carboxypeptidases A and B degrade peptides into amino acids in the small intestine
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Dietary protein is enzymatically degraded through the digestive tract
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Overview of Amino Acid Catabolism
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Removal of the Amino Group
First step of degradation for all amino acids
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Fates of Nitrogen in Organisms • Plants conserve almost all the nitrogen
• Many aquatic vertebrates release ammonia to their environment – Passive diffusion from epithelial cells
– Active transport via gills
• Many terrestrial vertebrates and sharks excrete nitrogen in the form of urea – Urea is far less toxic that ammonia
– Urea has very high solubility
• Some animals such as birds and reptiles excrete nitrogen as uric acid – Uric acid is rather insoluble
– Excretion as paste allows the animals to conserve water
• Humans and great apes excrete both urea (from amino acids) and uric acid (from purines)
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Excretory Forms of Nitrogen
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Enzymatic Transamination
• Catalyzed by aminotransferases • Uses the pyridoxal phosphate cofactor • 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
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Enzymatic Transamination
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Structure of Pyridoxal Phosphate and Pyridoxamine Phosphate
• Intermediate, enzyme-bound carrier of amino groups • Aldehyde form can react reversibly with amino groups • Aminated form can react reversibly with carbonyl groups
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Pyridoxal phosphate is covalently linked to the enzyme in the resting enzyme
• By an internal aldimine
• The linkage is made via a
nucleophilic attack of the
amino group of an
active-site lysine
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Internal Aldimine in Aminotransferases
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Chemistry of the Amino Group Removal by the Internal Aldimine
The external aldimine of PLP is a good electron sink, allowing removal of -hydrogen
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PLP also catalyzes racemization of amino acids
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PLP also catalyzes decarboxylation of
amino acids
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• Oxidative deamination occurs within mitochondrial matrix
• Can use either NAD+ or NADP+ as electron acceptor
• Ammonia is processed into urea for excretion
• Pathway for ammonia excretion; transdeamination = transamination + oxidative deamination
Ammonia collected in glutamate is removed by glutamate dehydrogenase
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Ammonia is safely transported in the bloodstream as glutamine
Excess glutamine is processed in intestines, kidneys, and liver.
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Glutamate can donate ammonia to pyruvate to make alanine
• Vigorously working muscles operate nearly anaerobically
and rely on glycolysis for energy
• Glycolysis yields pyruvate
– if not eliminated lactic acid will build up
• This pyruvate can be converted to alanine for transport
into liver
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The Glucose-Alanine Cycle
Alanine serves as a
carrier of ammonia
and of the carbon
skeleton of pyruvate
from skeletal muscle
to liver.
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Excess glutamate is metabolized in the mitochondria of hepatocytes
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Ammonia is highly toxic and must be utilized or excreted
• Free ammonia released from glutamate is converted to
urea for excretion.
• Carbamoyl phosphate synthase I captures free ammonia
in the mitochondrial matrix
• First step of the urea cycle
• Regulated
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Ammonia is recaptured via synthesis of carbamoyl phosphate
• The first nitrogen-acquiring reaction of the urea cycle
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Nitrogen from carbamoyl phosphate enters the urea cycle
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The Reactions in the Urea Cycle
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Entry of Aspartate into the Urea Cycle This is the second nitrogen-acquiring reaction.
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Aspartate –arginosuccinate shunt links urea cycle and citric acid cycle
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Regulation of the Urea Cycle
• Carbamoyl phosphate synthase I is activated by N-acetylglutamate
• Formed by N-acetylglutamate synthase
– When glutatmate and acetyl-CoA concentrations are high
– Activated by arginine
• Expression of urea cycle enzymes increases when needed
– High protein diet
– Starvation, when protein is being broken down for energy
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Regulation of the Urea Cycle
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Not all amino acids can be synthesized in humans
• Essential amino acids must be obtained as dietary protein
• Consumption of a variety of foods supplies all the essential amino acids
– including vegetarian- only diets
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End products of Amino Acid Degradation
• Intermediates of the Central Metabolic Pathway • Some amino acids result in more than one intermediate
• Ketogenic amino acids can be converted to ketone bodies
• Glucogenic amino acids can be converted to glucose
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
Seven to Acetyl-CoA Leu, Ile, Thr, Lys, Phe, Tyr, Trp
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Summary of Amino Acid Catabolism
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Several cofactors are involved in amino acid catabolism
• Important in one-carbon transfer reactions
– Tetrahydrafolate (THF)
– S-adenosylmethionine (adoMet)
– Biotin
• Biotin, as we saw in Chapter 16, transfers CO2
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THF is a versatile cofactor
• Tetrahydrofolate is formed from folate
– an essential vitamin
• THF can transfer 1-carbon in different oxidation states
– CH3, CH2OH, and CHO
• Used in a wide variety of metabolic reactions
• Carbon generally comes from serine
• Forms interconverted on THF before use
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THF is a versatile cofactor
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adoMet is better at transferring CH3
• S-adenosylmethionine is the prefered cofactor for methyl transfer in biological reactions – Methyl is 1000 times more reactive than THF methyl group
• Synthesized from ATP and methionine • Regeneration uses N5-methyl THF
– The only known use in mammals
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Activated Methyl Cycle
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Degradation of ketogenic amino acids
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Degradation intermediates of tryptophan are to synthesize other molecules
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Genetic defects in many steps of Phe degradation lead to disease
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Phenylketonuria is caused by a defect in the first step of Phe degradation
• A buildup of phenylalanine and phenylpyruvate
• Impairs neurological development leading to intellectual deficits
• Controlled by limiting dietary intake of Phe
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Degradation of Amino acids to Pyruvate
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Degradation of Glycine
• Pathway #1: hydroxylation to serine pyruvate
• Pathway #2: Glycine cleavage enzyme – Apparently major pathway in mammals – Separation of three central atoms – Releases CO2 and NH3 – Methylene group is transferred to THF
• Pathway #3: D-amino oxidase – Relatively minor pathway – Ultimately oxidized to oxalate – Major component of kidney stones
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Degradation of Amino Acids to α-Ketoglutarate
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Degradation of branched chain amino acids does not occur in the liver
• Leucine, Isoleucine, and Valine are oxidized for fuel – In muscle, adipose tissue, kidney, and brain
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Degradation of Asn and Asp to Oxaloacetate
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