Class Note for BIOC 460 at UA 5
Class Note for BIOC 460 at UA 5
Popular in Course
Popular in Department
This 8 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Arizona taught by a professor in Fall. Since its upload, it has received 10 views.
Reviews for Class Note for BIOC 460 at UA 5
Report this Material
What is Karma?
Karma is the currency of StudySoup.
You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!
Date Created: 02/06/15
Biochemistry 460 Lecture 33 7 Dr Tischler NITROGEN METABOLISM Related Reading Chapter 23 656674 in Stryer 6th edition Chapter 24 685693 702705 in Stryer 639h edition OBJECTIVES 1 Describe the transaminase and glutamate dehydrogenase reactions and discuss their roles in the removal of nitrogen waste in the body 2 Describe the urea cycle 3 Describe the phenylalanine hydroxylase reaction and explain its relationship to phenylketonuria 4 List the steps in the conversion of heme to conjugated bilirubin 5 De nejaundice PHYSIOLOGICAL PREMISE Have you ever carefully read a packet of EqualTM If so you may have noticed a warning to phenylketonurics The chemical sweetener in equal is a dipeptide containing phenylalanine and aspartate Some individuals are born with one of the more common amino acid disorders phenylketonuria They are unable to metabolize phenylalanine to tyrosine Consequently vast amounts of phenylalanine will accumulate in the blood if too much of this amino acid is consumed in the diet Constant excess of phenylalanine in the blood can cause severe mental retardation Hence this is one of several diseases tested for in newborns in all states PROTEIN TURNOVER AND NITROGEN ECONOMY Whether from exogenous sources beef sh milk or from endogenous sources myosin actin albumin protein metabolism strikes a balance between the body s energy and synthetic needs A certain amount of dietary protein is required daily to synthesize endogenous proteins such as albumin myosin actin and hemoglobin The basis for the dietary protein requirement is the inability of the body to synthesize certain amino acids these are termed essential amino acids The remaining amino acids nonessential can be synthesized from a variety of carbon sources The essential and nonessential amino acids are summarized in Table 1 Table 1 The essential and nonessential amino acids Essential Nonessential Argininea Methionineb Alanine Glummine Histidine Phenylalaninec Aspartate Glycine Isoleucine Threonine Asparagine Proline Leucine Tryptophan Cysteine Serine Lysine Valine Glutamate Tyrosine aArginine is synthesized in the urea cycle but the rate is too slow to meet the needs of growth in children bMethionine is required to produce cysteine if the latter is not supplied adequately by the diet cPhenylalanine is needed in larger amounts to form tyrosine if the latter is not supplied by the diet Nitrogen Metabolism l Protein balance describes the relationship between the synthesis and degradation proteolysis of proteins Fig l Proteolysis plays a wide variety of roles in the body In the course of previous lectures we have seen examples of these Proteolysis is used for activation of zymogens to their functional forms such as in the clotting cascade and in digestion A variety of proteases hydrolyze dietary proteins B ODY PROTEIN Proteolysis Protein synthesis gt De novo synthesis Catabolism Carbon compounds nitr0gen gt Ammo ACIdPOOI quotnununquot U1 aCO2 Dietary amino acids E Biosynthesis of nitrogen compounds Porphyrins creatine carnitine hormones nucleotides gt Amino acid sources 39 Fates of amino acids Figure 1 Sources and fates of amino acids During starvation glucose is produced from amino acids so that muscle protein can indirectly serve as a fuel supply To provide proper balance during growth proteolysis counterbalances synthesis to control organ size Following injury damaged tissue must be removed Proteins with severe defects are routinely removed by proteolysis Finally the activity of some enzymes is controlled by physically removing the enzyme from the cell via proteolysis The turnover rate of proteins varies considerably with t1 2 values time required for 50 of a certain amount of protein to be degraded ranging from minutes in the case of a high degree of regulation to days for the stable proteins such as myosin to 4 months for hemoglobin If the dietary intake of amino acids as protein is greater than the requirement for protein synthesis either new body protein is synthesized positive balance or body protein levels are maintained at a stable level neutral balance Positive nitrogen balance occurs predominantly during growth when intake and storage of all nitrogen sources exceeds excretion of all types of nitrogenous compunds in urine and sweat If caloric or protein intake is insuf cient or if the balance of the amino acids ingested is incorrect for synthetic needs endogenous protein is catabolized to liberate free amino acids for synthesis of essential proteins negative nitrogen balance Negative nitrogen balance is associated with conditions such as starvation trauma or cancer cachexia Negative protein balance occurs when the rate of protein degradation proteolysis exceeds the rate of protein synthesis This can occur either due to a decreased rate of synthesis andor accelerated degradation The key difference between protein and nitrogen balance is that protein balance considers proteins only and is a speci c subset of nitrogenh balance that considers all nitrogenous compunds Nitrogen Metabolism 2 The interrelationship of amino acids between their various sources and fates is represented in Figure 1 Excess amino acids or those derived from endogenous proteins during caloric insuf ciency are catabolized to produce energy which is captured as ATP The elemental constituents of the amino acids carbon hydrogen nitrogen and sulfur have the following ultimate fates carbon is turned into C02 hydrogen to H20 nitrogen to urea or ammonia and sulfur to S0421 AMMONIA METABOLISM Aminotransferase Reactions The rst step in the degradation of most amino acids is removal of their amino nitrogen group in a reaction catalyzed by speci c aminotransferases also called transaminases Fig 2 These reactions involve amino group donors and amino group acceptors and require pyridoxal phosphate derived from vitamin B6 pyridoxine as a cofactor The amino group of the amino acid donor is transferred to the second carbon of an ocketoacid acceptor usually ocketoglutarate The amino acid is thus converted to an ocketoacid and the ocketoglutarate changes to glutamate These enzymes are freely reversible to allow for synthesis of most of the nonessential amino acids those that can be synthesized in the body In the reverse glutamate becomes the amino group donor and some ocketoacid the amino group acceptor Aminotransferases generally transfer nitrogen to glutamate in non hepatic tissues such as muscle as a way of getting rid of excess nitrogen from those tissues In contrast in liver nitrogen is dumped onto glutamate as an initial step in the conversion of nitrogen to a form that can be readily excreted as urea H2 ocAmino ac1d xKetoglutarate E HOOCCHR HOOC CHzCHzCOOH II 112 HOOCE R 39Ketquot acid Glutamate HOOCCH CHzCHzCOOH Figure 2 Depiction of a general transamination aminotransferase reaction The ocamino acid other than glutamate can be a wide variety The most ubiquitous of these aminotransferases are aspartate aminotransferase and alanine aminotransferase Activities of these enzymes are often measured in the blood for liver enzyme function tests Alanine aminotransferase is important in the liver for gluconeogenesis from alanine and in the muscle for producing large amounts of alanine for transport to liver Nitrogen Metabolism 3 Glutamate Dehydragenase Glutamate dehydrogenase serves different functions depending on the tissue in which it is found As a reduction reaction Fig 3 the enzyme catalyzes the addition of nitrogen to ocketoglutarate as ammonia in nonhepatic tissues This pathway for glutamate production is an important way to remove harmful ammonia from these tissues Since glutamate is not transported across the plasma membrane readily it undergoes an additional amination through the addition of a second ammonia molecule by glutamine synthetase to produce glutamine Fig 3 The glutamine is processed by the kidney which removes both amino groups to excrete the ammonia via the urine In liver the glutamate produced by transamination gives up its nitrogen as free ammonia via glutamate dehydrogenase for the eventual synthesis of urea which can be excreted Fig 4 The process of urea production is discussed below NADH NAD xKetoglutarate NH4 L LD Glutamate Glutamate dehydrogenase NH3ATP Glutamine synthetase ADP P Glutamine Figure 3 In nonhepatic tissues the linked reactions of glutamate dehydrogenase and glutamine synthetase remove two ammonia molecules from the tissues as a way of ridding the tissues of nitrogen waste The glutamine deposits the ammonia in the kidney for excretion NADH NH ocAmino acid xKetoglutarate V Jrea cycle Aminotransferase Glu dehydrogenase UREA ocKeto acid A Glutamate A N AD H20 Figure 4 In liver nitrogen waste from amino acids ends up in urea Amino acids are derived either from the breakdown of protein in various tissues or from what is synthesized in those tissues Nitrogen Metabolism 4 Urea Cycle HOOCCH if HzNC NH CHZCOOH Fumarate ZATP HCO3 NH4 returns to UREA TCA cycle C arbamoyl Argmm NHZ phosphate H93 synthetase HZNdHCOOH ZADP Pi O thl Arg1n1nosucc1nate Ornithine m1 ne Carbamoyl phosphate HZNC GPOng AMPPPi Ornithine transcarbamoylase Citrulline Citrulline Pi O MITOCHONDRIA 39oocc HNH ATP HNNHZ r H CHZCOO CYTOPLASM Z 3 Aspartate HZNCHCOOH Figure 5 The carbamoyl phosphate synthetase reaction reaction D and the urea cycle reactions to The boldfaced components comprise the sources of the two nitrogens and the carbon in urea The overall sum of the ve reactions of the urea cycle is 3ATP HCO339 NH aspartate 9 2ADP AMP 2P PP fumarate urea The urea cycle is found primarily in the liver and to a lesser eXtent in the kidney It provides a means of ridding the body of nitrogen waste as urea In liver ammonia is derived from amino acids by the combined actions of transamination and glutamate dehydrogenase Fig 6 In the mitochondria ammonia is incorporated into carbamoyl phosphate via carbamoyl phosphate synthetase I Fig 7 reaction D This enzyme is not directly part of the cycle but instead the product carbamoyl phosphate provides a substrate for the cycle The reaction is energy requiring with one ATP molecule providing the phosphate that combines with carbon dioxide and ammonia and the other ATP providing the driving force for the reaction Carbamoyl phosphate directly introduces the rst source of nitrogen for the cycle Carbamoyl phosphate synthetase I is allosterically activated by N acetylglutamate N acetylglutamate is synthesized from of acetyl CoA and glutamate by a synthetase reaction Ornithine transcarbamoylase reaction catalyzes the combination of carbamoyl phosphate with omithine to form citrulline This reaction occurs in the mitochondrial matriX putting it in the same compartment as the site of carbamoyl phosphate formation Ornithine for this reaction is transported into the mitochondria from the cytoplasm The citrulline product of the ornithine transcarbamoylase reaction is released from the mitochondria to the cytoplasm in exchange for ornithine Nitrogen Metabolism 5 In the cytoplasm citrulline reacts with aspartate via argininosuccinate synthetase reaction yielding argininosuccinate Aspartate for this reaction is formed by transamination of glutamate with oxaloacetate Thus aspartate is the second direct source of nitrogen for the cycle As a synthetase it is an energy requiring reaction that cleaves ATP to AMP PP thus costing two highenergy phosphate bonds recall that PP spontaneously splits to two Pi Argininosuccinate is cleaved by argininosuccinase into fumarate and arginine reaction GD The fumarate can be reconverted to oxaloacetate in the citric acid cycle that in turn can be used to regenerate aspartate Hence the carbons from aspartate are recycled with only the nitrogen claimed for the urea cycle This also provides an important interaction between the urea cycle and the citric acid cycle It is noteworthy that Sir Hans Krebs is credited for discovering both cycles the former in conjunction with Henseleit This is truly amazing because in the 1930 s he lacked any of the modern equipment we use for chemical analysis Arginase cleaves arginine into urea and ornithine reaction Urea is secreted from the liver into the blood to be cleared by the kidney for excretion Ornithine the other product is regenerated for another turn of the cycle H ypemmmtmemia Hyperammonemia is acquired in adults by development of severe liver damage cirrhosis Blood then bypasses the liver leading to a drastic reduction in ammonia detoxi cation by the liver Ammonia arising from amino acid or protein catabolism cannot be converted to urea to any signi cant extent causing blood ammonia to rise During an episode blood ammonia can increase more than 400fold Hyperammonemia may also be caused by inherited de ciencies of urea cycle enzymes almost solely seen in children The severity of inherited hyperammonemia depends on how close the defect is to the point of entry of ammonia into the cycle A total or near loss of carbamoyl phosphate synthetaseI or of ornithine transcarbamoylase is usually fatal to the newborn Most of these defects produce mental retardation since ammonia reacts with xketoglutarate to form glutamate there likely is a reduction in energy metabolism in the brain caused by depletion of citric acid cycle intermediates PHENY LALAN IN E METABOLISM There are many disorders directly linked to amino acid metabolism that are characterized by defects of enzymes in their pathways Such defects can either be caused by a nonfunctional enzyme or an enzyme that poorly binds a cofactor In the latter instance treatment of the patient with excess cofactor can partially overcome this binding dif culty One wellstudied disorder is phenylketonuria see physiological premise a defect of phenylalanine hydroxylase and the rst reaction in the degradation of phenylalanine Fig 6 Normally the body derives tyrosine as a product of this reaction however individuals with this disorder must consume suf cient amounts of tyrosine in the diet Phenylalanine hydroxylase has a requirement for tetrahydr0H4bi0pterin as a cofactor Therefore the defect can reside either in the hydroxylase itself or in metabolism of the cofactor As a consequence of this defect a variety of unusual compounds may be produced including phenylpyruvate and phenyllactate Phenylpyruvate is produced by the large amounts of phenylalanine being transaminated by alanine transaminase while the phenylpyruvate product is then metabolized further to phenyllactate or to phenylacetate Fig 6 A major consequence of this disorder is mental retardation The most likely explanation for this symptom is that the excessive phenylalanine in the blood interferes with a proper balance of amino acids being transported into the brain for synthesis of brain proteins Nitrogen Metabolism 6 primary defect in Phenylalanine hydroxylase phenylketonuria Oz H20 PhenylpymVate 4 Phenylalanine Tyrosine Tetrahydrobiopterin Dihydmbiopte n Phenyllactate Phenylacetate NADP NADPH Figure 6 Unusual compounds produced from phenylalanine in phenylketonuria The phenylalanine hydroxylase reaction or regeneration of the tetrahydrobiopterin cofactor are defective in phenylketonuria HEME CATABOLISM Heme proteins like all other proteins are constantly synthesized and degraded The major heme containing molecule is hemoglobin found in red blood cells When the red blood cell ends its life after 120 days the hemoglobin molecule is degraded Amino acids from globin and the iron are recycled but the porphyrin molecule is degraded Bilirubin is the end product of heme catabolism The pathway for bilirubin production is shown in Figure 7 Heme oxygenase converts the heme backbone to biliverdin IXoL This reaction is the only endogenous one known to produce carbon monoxide CO Biliverdin is then converted to bilirubin in a form uncanjugated 0r insoluble that is yellow poorly soluble in water and highly toxic Bilirubin is transported into the plasma where it binds to albumin and is transported to the liver In the liver bilirubin is made watersoluble by conjugation with two molecules of glucuronic acid Bilirubin diglucuronide conjugated 0r soluble bilirubin is excreted into the bile duct and into the intestine In the lumen of the intestine bacterial enzymes act on the conjugated bilirubin to produce urobilinogen A small amount of urobilinogen is absorbed by intestinal cells and sent via the blood to the kidney where it is excreted in the urine as urobilin Most of the urobilinogen in the intestine is oxidized to stercobilin which is the major excreted product from heme and imparts the brown color to feces Nitrogen Metabolism 7 BLOOD CELLS Stercobilin Hemoglobin excreted in feces TJrobilln excreted 1n urlne N Globin Heme O Urob111nogen 2 gt Heme formed by bacteria reabsorbed into blood oxygenase INTESTINE CO via bile duct to intestines Biliverdin IXoc NADPH Bilirubin diglucuronide BlllVCrdlIl water soluble conjugated reductase NADP 2 UDPglucuronic acid Bilirubin gigggitnsoluble39 uncon 39u ated water insoluble J g unconjugated via blood to the liver LIVER Figure 7 Catabolism of hemoglobin Heme is removed from hemoglobin and converted to insoluble bilirubin in the red blood cell Bilirubin is then solubilized in the liver by adding two glucuronic acid molecules Bilirubin is further processed in the intestine and some in the kidney for excretion as waste Medical Scenario The van den Bergh test measures blood levels of conjugated and unconjugated bilirubin The basis of this blood test depends on the solubility of these two forms Unconjugated bilirubin is water insoluble whereas conjugated bilirubin is water soluble In the van den Bergh analysis bilirubin is coupled with diazonium salts to produce colored azo dyes Conjugated bilirubin diglucuronide is readily solubilized in water and yields color via a direct reaction The unconjugated bilirubin must rst be eXtracted with ethanol before reaction with diazonium salts and this is hence is considered to be an indirect reaction Clinically the terms direct hyperbilirubinemia and indirect hyperbilirubinemia are used to signify elevations of conjugated and unconjugated bilirubin respectively The term mixed means elevation of both forms Jaundice is the term used to clinically describe the yellow color seen in hyperbilirubinemia and is due to deposition of the bilirubin in the skin and conjunctiva Nitrogen Metabolism 8
Are you sure you want to buy this material for
You're already Subscribed!
Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'