Nitrogen Metabolism PDF

NITROGEN METABOLISM Learning objectives:
Describe amino acids pool and protein turnover in nitrogen metabolism
Describe how are proteins being digested and absorbed in the body
State the overall catabolism of amino acids – transamination and deamination
Explain how amino-nitrogen is tr...

NITROGEN METABOLISM Learning objectives:
Describe amino acids pool and protein turnover in nitrogen metabolism
Describe how are proteins being digested and absorbed in the body
State the overall catabolism of amino acids – transamination and deamination
Explain how amino-nitrogen is transformed to urea for excretion
Illustrate the conversion of the carbon skeleton from protein to glucose or ketone bodies
Describe the synthesis of nonessential amino acids 
Unlike fats and carbohydrates, amino acids are not stored by the body
Amino acids must be obtained  from the diet,  synthesized de novo, or  produced from normal protein degradation.
Any amino acids in excess of the biosynthetic needs of the cell are rapidly degraded. OVERALL NITROGEN METABOLISM
Amino acid catabolism is part of the larger process of the metabolism of nitrogen-containing molecules.
Nitrogen enters the body : food
Nitrogen leaves the body : urea, ammonia, and other products derived from amino acid metabolism.
The role of body proteins in these transformations involves two important concepts:  the amino acid pool and  protein turnover. Amino acid pool
Amino acid pool is supplied by three sources: 1) amino acids provided by the degradation of body proteins, 2) amino acids derived from dietary protein, and 3) synthesis of nonessential amino acids from simple intermediates of metabolism.
Amino pool is depleted by three routes: 1) synthesis of body protein, 2) amino acids consumed as precursors of essential nitrogen- containing small molecules, and 3) conversion of amino acids to glucose, glycogen, fatty acids, ketone bodies, or CO2 + H2O. DIGESTION OF DIETARY PROTEINS
Proteins are generally too large to be absorbed by the intestine. [Exception: newborns can take up maternal antibodies in breast milk.]
Proteins must, therefore, be hydrolysed to yield di- and tripeptides as well as individual amino acids, which can be absorbed.
Proteolytic enzymes responsible for degrading proteins are produced by three different organs: i. the stomach, ii. the pancreas, and iii. the small intestine A. Digestion of proteins by gastric secretion
Stomach -- gastric juice containing hydrochloric acid and the proenzyme, pepsinogen 1. Hydrochloric acid: Functions  To kill some bacteria and  to denature proteins,
thus making them more susceptible to subsequent hydrolysis by proteases. 2. Pepsin:
This acid-stable endopeptidase is secreted by the chief cells of the stomach as an inactive zymogen (or proenzyme), pepsinogen.
Pepsinogen is activated to pepsin, either by HCl, or autocatalytically by other pepsin molecules that have already been activated.
Pepsin releases peptides and a few free amino acids from dietary proteins. B. Digestion of proteins by pancreatic enzymes
Large polypeptides produced in the stomach by the action of pepsin are further cleaved to oligopeptides and amino acids by a group of pancreatic proteases.
Proteases:
has different specificity for the amino acid R-groups adjacent to the susceptible peptide bond
are synthesized and secreted as inactive zymogens
The release and activation of the pancreatic zymogens is mediated by the secretion of cholecystokinin and secretin Cleavage of dietary protein by proteases from the pancreas C. Digestion of oligopeptides by enzymes of the small intestine
The luminal surface of the intestine contains aminopeptidase—an exopeptidase  that repeatedly cleaves the N-terminal residue from oligopeptides  to produce even smaller peptides and free amino acids. Digestion of dietary proteins by the proteolytic enzymes of the gastrointestinal tract.  2 CHOICES 1. Reuse 2. Urea cycle Fumarate Oxaloacetate Overview of amino acid catabolism in mammals Nitrogen removal from amino acids Transamination Aminotransferase PLP Oxidative deamination Urea cycle Nitrogen removal from amino acids Step 1: Remove amino group Step 2: Take amino group to liver for nitrogen excretion Step 3: Entry into mitochondria Step 4: Prepare nitrogen to enter urea cycle Step 5: Urea cycle REMOVAL OF NITROGEN FROM AMINO ACIDS
Removing the α-amino group is essential for producing energy from any amino acid, and is an obligatory step in the catabolism of all amino acids.
Once removed, this nitrogen can be incorporated into other compounds or excreted, with the carbon skeletons being metabolized. (A) Transamination: the funnelling of amino groups to glutamate
First step in amino acid catabolism:  transfer of α-amino group to α-ketoglutarate
Enzyme:  Aminotransferase  transaminases
Yield products: 1. an α-keto acid (derived from the original amino acid) and Oxidative deamination amino group donor in the synthesis 2. glutamate of nonessential amino acids. 
Gluatamate’s amino group, in turn, can be transferred to oxaloacetate in a second transamination reaction, yielding aspartate and reforming α-ketoglutarate.
Coenzyme:
Pyridoxal-5’-phosphate (PLP) 1st transamination 2nd transamination Aminotransferases – Alanine aminotransferases (ALT) Aspartate aminotransferases (AST) During amino acid catabolism, ALT functions in the direction of glutamate synthesis.  During amino acid catabolism, AST transfers amino groups from glutamate to oxaloacetate, forming aspartate All aminotransferases require the coenzyme pyridoxal phosphate (a derivative of vitamin B6) 
Transamination  does not result in any net deamination
Glutamate can be oxidatively deaminated by glutamate dehydrogenase
End products:  Ammonia which is a source of nitrogen in urea synthesis.  Regenerating α-ketoglutarate (B) Oxidative deamination by glutamate dehydrogenase
results in the release of the amino group as free ammonia (NH3)
occur primarily in the liver and kidney  Oxidative deamination by glutamate dehydrogenase Oxidation - transfer of a hydride ion from glutamate’s Cα to NAD(P)+,  forming iminoglutarate, - which is hydrolysed to α-ketoglutarate and ammonium ion  Glutamate dehydrogenase
Glutamate is unique in that it is the only amino acid that undergoes rapid oxidative deamination. Coenzymes:
Glutamate dehydrogenase is unusual in that it can use either NAD+ or NADP+ as a coenzyme 
α-Ketoglutarate  Intermediate of TCA cycle
Activation of glutamate dehydrogenase Stimulate flux through the TCA cycle, -- leading to increased ATP production Glutamate dehydrogenase
Allosterically inhibited by GTP and NADH  Signaling abundant metabolic energy
activated by ADP and NAD+  Signaling the need to generate energy Direction of reactions
Depends on
the relative concentrations of glutamate, α-ketoglutarate, and ammonia,
the ratio of oxidized to reduced coenzymes
Guanosine triphosphate (GTP)
allosteric inhibitor of glutamate dehydrogenase
adenosine diphosphate (ADP)
Activator of glutamate dehydrogenase Describe how α-ketoglutarate and oxaloacetate participate in amino acid catabolism. Transport of ammonia to the liver Entry of nitrogen to mitochondria Prepare nitrogen to enter urea cycle Regulation Excretion of excessive nitrogen in the form of: UREA CYCLE Urea
One nitrogen of the urea molecule is supplied by free ammonia, and the other nitrogen by aspartate
The carbon and oxygen of urea are derived from CO2
Urea is produced by the liver, and then is transported in the blood to the kidneys for excretion in the urine. The overall Urea Cycle reaction is Urea Cycle: Reactions of the cycle 1. Formation of carbamoyl phosphate 2. Formation of citrulline 3. Synthesis of argininosuccinate 4. Cleavage of argininosuccinate 5. Cleavage of arginine to ornithine and urea Urea cycle.pdf  Irreversible Rate-limiting enzyme (1) Formation of carbomoyl phosphate Carbamoyl phosphate synthetase (CPS) catalyses the condensation and activation of NH3 and HCO3- to form carbomoyl phosphate (2) Formation of citrulline Carbamoylation of ornithine produces citrulline Enzyme: Ornitine transcarbamoylase Transfers the carbamoyl group of carbamoyl phosphate to ornithine, yielding citrulline (3) Synthesis of argininosuccinate Argininosuccinate synthetase acquires the second urea nitrogen atom. (4) Cleavage of argininosuccinate Argininosuccinase produces fumarate and arginine (5) Cleavage of arginine to ornithine and urea By arginase The urea cycle is regulated by substrate availability CPS I is allosterically activated by N-acetylglutamate N-acetylglutamate is synthesized from glutamate and acetyl CoA by N-acetylglutamate synthetase. When does the glutamate concentration increases? When amino acid breakdown rates increase [glutamate] ↑↑  synthesis of N-acetylglutamate ↑↑ N-acetylglutamate ↑↑  CPS I acƟvity ↑↑ Checkpoint 1. How do the amino groups of amino acids enter the urea cycle? Breakdown of amino acids Amino acids are degraded to compounds that can be metabolized to CO2 and H2O or used in gluconeogenesis. Amino acids are degraded to one of seven metabolic intermediates: 1. Pyruvate 2. α-Ketoglutarate 3. Succinyl-coA 4. Fumarate 5. Oxaloacetate 6. Acetyl-CoA 7. acetoacetate  The amino acids can therefore be divided into two groups based on their catabolic pathways: Glucogenic amino acids Ketogenic amino acids Amino acids whose carbon Amino acids whose carbon skeletons are degraded to skeletons are broken down to glucose precursors acetyl CoA or acetoacetate and Glucose precursors: can thus be converted to fatty pyruvate, acids or ketone bodies α-Ketoglutarate Succinyl-coA Fumarate Oxaloacetate Degradation of amino acids to one of seven common metabolic intermediates Green: glucogenic degradation Red: Ketogenic degradation Amino Acid Biosynthesis
Essential amino acid  cannot be synthesized in sufficient quantities for growth and maintenance and, therefore, are required in diet. The Essential Amino Acids are:  Phenylalanine, Phe  Valine,Val  Threonine,Thr  Tryptophan,Trp  Isoleucine,Ile  Methionine,Met  Histidine,His  Arginine,Arg *only essential in children, not adults  Leucine,Leu  Lysine,Lys PVT TIM HALL (i.e. Private Tim Hall). Nonessential amino acids are synthesized from common metabolites
Four common metabolic intermediates: 1. Pyruvate 2. Oxaloacetate 3. α-Ketoglutarate 4. 3-phosphoglycerate Only tyrosine is synthesized by one-step hydroxylation of phenylalanine (essential amino acid)  Transamination Amidation BIOSYNTHESIS OF NONESSENTIAL AMINO ACIDS 1.Transamination of α-keto acids The conversion of 3-phosphoglycerate to serine - Transamination 2. Synthesis by amidation 2. Synthesis by other amino acids Amino acids Metabolism Learning objectives:
Describe amino acids pool and protein turnover in nitrogen metabolism
Describe how are proteins being digested and absorbed in the body
State the overall catabolism of amino acids – transamination and deamination
Explain how amino-nitrogen is transformed to urea for excretion
Illustrate the conversion of the carbon skeleton from protein to glucose or ketone bodies
Describe the synthesis of nonessential amino acids