1 MB ChB PHASE I HOW THE BODY HANDLES NITROGEN LECTURE 1 AIM: To review: N-containing molecules of the body; N flow through the biosphere; N flow through the body; digestion of dietary protein; role of glutamate in transfer of N to, from and between amino-acids. [C&H Chapters 19, 20; pp.367-369] http://www.abdn.ac.uk/~bch118/index.htm 2 N-CONTAINING THE BODY MOLECULES OF The main ones are: 1 Proteins (contain amino-acids) 2 Nucleic acids and nucleotides (contain bases) 3 Active amines CO2 amino-acid 1 /2O2 + NH4 amine aldehyde ACTIVE INACTIVE (several, non-coded) INACTIVE 3 eg histamine serotonin dopamine (nor-)epinephrine. 4 Haem-containing cytochromes. haemoglobin and (Proteins and Terminal Respiration lectures) 4 N FLOW THROUGH THE BIOSPHERE N in biomolecules comes ultimately from N2 (80% of the atmosphere). However, N2 isn’t readily exchanged with N found in organisms. (Compare with ready exchange environmental and biological C, H, O.) between This is because N2 is very unreactive: N2 is N N. Only a few micro-organisms can reduce N2 and ‘fix’ it in the biosphere. Once ‘fixed’, as NH4+, N flows to other biomolecules through the (coded) aminoacid glutamate. [For structure, see Proteins Lecture 2.] Only a few (different) micro-organisms can pass N back to the atmosphere, as N2. 5 ATMOSPHERE N2 BIOSPHERE + NH4 glutamate (other) amino-acids proteins and other N-containing biomolecules amino-acids glutamate + NH4 N2 6 N FLOW THROUGH THE BODY The main flow involves: 1 digestion of dietary protein; ‘turn-over’ of body protein; excretion of N as urea; 2 a pool of intra- and extra-cellular free amino-acids (kept constant at about 100g). 7 body protein dietary protein synthesis of some (‘non-essential’) amino-acids amino-acid pool body protein synthesis of other N-containing molecules N excreted as urea C or ATP glucose (gluconeogenesis) fatty acids ketone bodies CO2 8 DIETARY PROTEIN Dietary protein replenishes amino-acids used in anabolism and catabolism. (as shown in the previous diagram) Normally, N flow into and out of the body is balanced. N flowin that is less than N flowout results in ‘negative nitrogen balance’. This happens in the condition kwashiorkor. [‘Sickness of the older child when the next baby is born’] It occurs when carbohydrate intake is adequate, but N intake is poor, as sometimes happens when a child is weaned onto a starchy diet. 9 Among several consequences: low N intake low plasma albumin low plasma osmotic pressure lower than usual entry of interstitial water into plasma oedema plump belly N flowin that is greater than N flowout results in ‘positive nitrogen balance’. This occurs during rapid tissue growth: eg in children; pregnancy; recovery from illness; body-building. N taken in in excess of requirements cannot be stored (unlike carbohydrate, fat) and is excreted. 10 Of the 20 coded amino-acids, 10 (‘non-essential’) can be synthesised in the body (they have simple R groups); 8 (‘essential’) cannot be synthesised and must be present in the diet; 2 (arginine, histidine) can usually be made at adequate rates, but are needed in the diet when tissue growth is rapid. Animal protein is usually rich in all essential amino-acids. An exception is collagen (in gelatin). [Proteins Lecture 4] Particular plant proteins are deficient in some essential amino-acids. Vegetarians need to use a mix of protein sources. 11 DIGESTION OF DIETARY PROTEIN 1 Stomach HCl denatures proteins and makes them accessible to degradative enzymes. The zymogen pepsinogen is cleaved to pepsin autocatalytically, and, later, by pepsin itself. Pepsin cleaves proteins to small polypeptides. 2 Small intestine Mucosal cell-surface enteropeptidase cleaves trypsinogen (secreted by the pancreas) to trypsin. Trypsin cleaves other pancreatic zymogens to elastase [met lung elastase in Proteins Lecture 4]; chymotrypsin; carboxypeptidases A and B. 12 These enzymes have different specificities: they cleave adjacent to different amino-acids. Together, they break polypeptides to free amino-acids and short peptides. In addition, a mucosal cell-surface aminopeptidase removes amino-acids one at a time from N termini. Amino-acids and short peptides are absorbed into mucosal cells by several methods, including ATP-driven Na+-dependent transport like that used for Glc. [Energy Transformations - Carbohydrates Lecture 2] Absorbed peptides are broken to amino-acids by a cytosolic peptidase. Amino-acids move through the portal system to the liver and are either metabolised directly or released into the general circulation. 13 body protein dietary protein synthesis of some (‘non-essential’) amino-acids amino-acid pool body protein synthesis of other N-containing molecules N excreted as urea C or ATP glucose (gluconeogenesis) fatty acids ketone bodies CO2 14 ROLE OF GLUTAMATE IN TRANSFER OF N TO, FROM AND BETWEEN AMINOACIDS Glutamate (Glu) is one of the 20 coded amino-acids. (Proteins Lecture 2] It forms a link in the flow of N between +NH4 and other amino-acids (see earlier ‘Biosphere’ diagram), and from one amino-acid to another. It does this by taking part in 2 reactions: 15 1 oxidative deamination NAD(P)H + H+ + NH4 + -ketoglutarate NAD(P)+ Glu + (KG) COO- COO- (CH2)2 (CH2)2 C O COO- HC NH2 COO- KG is a citric acid cycle intermediate. [Terminal Respiration Lecture 1] H2O 16 The reaction: can use either NAD or NADP; can proceed in either direction (depending on the conditions); is an oxidative deamination when it proceeds R L; is catalysed by (named for the R glutamate dehydrogenase L reaction); is crucial to life: Glu is the ONLY amino-acid that can obtain its N directly from +NH4 (in the L R reaction). 17 All others obtain their N from a pre-existing amino-acid (as in the following reaction). 2 transamination (aminotransfer) Glu eg + -ketoacid pyruvate CH3 C O COO- -KG + amino-acid alanine CH3 HC NH2 COO- and oxaloacetate aspartate COO- COO- CH2 CH2 C O COO- HC NH2 COO- 18 These reactions: can proceed in either direction (depending on the conditions); are catalysed by transaminases (aminotransferases). use pyridoxal phosphate (made from vitamin B6) as a cofactor. Measurement of these normally intracellular enzymes in plasma allows progression of liver and heart disease (in which cell damage and enzyme leakage occur) to be followed. 19 In summary: Glu, through these two reactions, plays a central role in: 1 movement of N from +NH4 to amino-acids + NH4 KG amino-acid Glu -ketoacid reaction 1 reaction 2 2 movement of N from amino-acids to +NH4 amino-acid KG -ketoacid Glu reaction 2 + NH4 reaction 1 20 3 transfer of N from one amino-acid to another amino-acid 1 KG amino-acid 2 -ketoacid 1 Glu -ketoacid 2 reaction 2 reaction 2 21 MB ChB PHASE I HOW THE BODY HANDLES NITROGEN LECTURE 2 AIM: To review: catabolism of body protein; transport of N to the liver; formation of urea for excretion. [C&H Chapters 19, 20; pp.367-369] 22 CATABOLISM OF BODY PROTEIN This is part of the N flow through the body that we saw in Lecture 1: body protein dietary protein synthesis of some (‘non-essential’) amino-acids amino-acid pool body protein synthesis of other N-containing molecules N excreted as urea C or glucose (gluconeogenesis) fatty acids ketone bodies ATP CO2 23 OVERVIEW OF THE FLOW OF N FROM CATABOLISED PROTEIN TO UREA body protein 1 amino-acids 2 Glu 3 + NH4 4 glutamine transported through plasma to liver 5 + NH4 + Glu 6 or 7 aspartate + NH4 urea transported through plasma to kidney for excretion 24 Also, in skeletal muscle: 2 Glu 8 alanine transported through plasma to liver 9 Glu 10 aspartate or + 11 NH4 25 FLOW OF N FROM PROTEIN TO UREA CATABOLISED 1 300-400g body protein is ‘turned-over’ daily. It is broken to amino-acids by intracellular proteinases. 2 Amino-acid -amino group N transferred to Glu by transamination. (as seen in Lecture 1) amino-acid + KG N Glu + -ketoacid N 3 Glu is oxidatively deaminated. (as seen in Lecture 1) Glu N KG + + NH4 N is 26 4 Ammonia is very toxic. Hyperammonaemia (high plasma [+NH4]) causes tremors; speech slurring; coma; death. Probably, high [+NH4] pulls the oxidative deamination reaction [Step 3] towards Glu synthesis KG, a component of the citric acid cycle, is depleted ATP production by oxidative phosphorylation decreases brain is vulnerable to low [ATP]. 27 Also, perhaps high [+NH4] depletes ATP by excessive flow through the glutamine synthetase reaction [see below]. And Glu and its derivative -aminobutyrate are concerned with neurotransmission: perhaps changes in their concentrations when [+NH4] is high also contribute to the symptoms. Because of ammonia toxicity, N is transported from many body tissues through plasma as non-toxic glutamine [one of the 20 coded amino-acids]. ATP + NH4 N ADP + Pi glutamine synthetase glutamine 2N Glu N COO(CH2)2 CONH2 (CH2)2 28 5 In the liver, glutamine is converted back to + NH4 and Glu. H 2O + glutamine NH4 + Glu N N glutaminase 2N 6 Glu now either passes its N to aspartate by transamination (as seen in Lecture 1), Glu + oxaloacetate N KG + aspartate N 7 or is oxidatively deaminated [as in step 3]. Glu N KG + + NH4 N 29 The N of the +NH4 and aspartate formed in the liver in steps 5-7 becomes N of urea. Skeletal muscle can also transport N from its degraded proteins through plasma as alanine. [See the ‘Overview’ diagram] 8 Glu, instead of being deaminated [as in step 3], is transaminated to alanine oxidatively [as seen in Lecture 1]. Glu N + pyruvate KG + alanine N Non-toxic alanine is now transported through plasma. 30 9 In the liver, alanine is transaminated back to Glu. [reverse of step 8] 10 Glu is then either transaminated to aspartate. [as in step 6] 11 or oxidatively deaminated to give +NH4. [as in step 7] As before, the N of the +NH4 and aspartate formed in the liver in steps 10 and 11 becomes N of urea. 31 Why transport N as glutamine or alanine, rather than Glu? Glu has a net negative charge: its transport would mean additional transport of a cation; also, its charge means it does not readily pass through membranes. Alanine and glutamine bear no net charge. [The reason why alanine in particular is transported from skeletal muscle is discussed in Lecture 3.] 32 FORMATION OF UREA FOR EXCRETION N from catabolised protein is now in the form of aspartate and +NH4 in the liver. It is transformed into N of urea. 33 The urea cycle is shown in C&H, p. 254. 34 To make it simpler: 2N (as urea) 1N (from +NH4 via carbamoyl phosphate) 2N 4N 3N 4N 1N (from aspartate) 35 Another version: [1N] [1C] + NH4 + CO2 2ATP 2ADP + Pi NH2 CO O OPO O H2N C O H2N urea carbamoyl phosphate [2N, 1C] ornithine arginine citrulline aspartate [1N] fumarate ATP AMP + PPi arginosuccinate 36 Arginine is one of the 20 coded amino-acids; ornithine and citrulline are uncoded aminoacids. Carbamoyl phosphate and citrulline syntheses occur in the mitochondrial matrix; the other reactions are cytosolic. Note the energy expended in excreting N as urea. Urea passes from the liver through plasma to the kidney for excretion. 37 Returning to hyperammonaemia: ‘Acquired hyperammonaemia’ may occur during liver malfunction, as in cirrhosis caused by alcoholism; hepatitis; bile duct obstruction. In such conditions, metabolism of N in the liver is prevented, and [plasma +NH4] increases. ‘Hereditary hyperammonaemia’ occurs when one or other of the urea cycle enzymes is genetically deficient. 38 Treatment includes: 1 lowering (but not eliminating) protein intake, to reduce load on the urea cycle. 2 iv benzoate glycine iv phenylbutyrate Gln Glycine and Gln synthesis, to replenish that removed, helps reduce [plasma +NH4]. non-toxic, readily excreted products 39 3 Partial deficiency in a urea cycle enzyme may be treated by specific supplementations /limitations in diet. Total deficiency of an enzyme is likely to be fatal. Kidney failure can hyperammonaemia. also result in A little liver urea passes (quite normally) to the gut lumen. Most is broken to +NH4 by bacteria. Some of this is lost in faeces; some moves into plasma. In kidney failure, more urea than usual passes to the gut, and more than usual +NH4 produced there moves into plasma. Treatment: oral antibiotics to inhibit bacterial synthesis. + NH4 40 MB ChB PHASE I HOW THE BODY HANDLES NITROGEN LECTURE 3 AIM: To review: fate of the C of catabolised body protein; why alanine is transported from muscle; removal of glutamine by the kidney; phenylketonuria. [C&H Chapters 19, 20; pp.367-369] 41 FATE OF THE C OF CATABOLISED BODY PROTEIN Refer back to the N flow through the body that we saw in Lecture 1: body protein dietary protein synthesis of some (‘non-essential’) amino-acids amino-acid pool body protein synthesis of other N-containing molecules N excreted as urea C or glucose (gluconeogenesis) fatty acids ketone bodies ATP CO2 42 and recollect the opening steps of the flow of N from catabolised protein to urea that we saw in Lecture 2. They were: body protein 1 amino-acids 2 2 Glu 3 + NH4 Glu or, in skeletal muscle, 8 alanine 43 Step 2 was the transamination: amino-acid + KG N Glu + -ketoacid N This can also be represented thus: KG amino-acid N and C Glu N -ketoacid C We traced the flow of N (eventually to urea). What happens to the C? 44 C of amino-acids is converted into the C of intermediates of: glycolysis/gluconeogenesis; citric acid cycle; lipid metabolism pathways. Sometimes the conversion is simple: eg the C of alanine and aspartate, by transamination, becomes the C of pyruvate and oxaloacetate respectively [as we saw in Lecture 1]. For other amino-acids, the conversion involves several steps. 45 To summarise, the flow for the various C atoms of the various amino-acids is: phosphoenol pyruvate pyruvate acetyl CoA oxaloacetate acetoacetate citrate fumarate succinylCoA KG 46 Any of these flows allows C atoms to be metabolised to CO2 through terminal respiration, providing ATP by oxidative phosphorylation. Those C atoms that flow to pyruvate or citric acid cycle intermediates [unfilled arrows in the diagram] can also generate Glc by gluconeogenesis. This occurs at a high rate during starvation, when glycogen (and triacylglycerol) stores are depleted, and muscle protein is degraded to provide Glc . [Proteins Lecture 1 (Protein Function 7); Terminal Respiration Lecture 1] 47 [Remember, from Terminal Respiration Lecture 1, when a citric acid cycle intermediate is produced, as in the scheme above, the ‘feed-in’ reaction allows a section of the cycle to become part of a linear pathway, and contribute to anabolism (here, of Glc).] In contrast, those C atoms that flow to acetyl CoA or acetoacetate [filled arrows in the diagram], can flow to fatty acids and (other) ketone bodies, but are not gluconeogenic, for the reasons given in Terminal Respiration Lecture 1: pyruvate dehydrogenase-catalysed irreversible; reaction is there is no net oxaloacetate synthesis through the citric acid cycle. 48 Amino-acids providing pyruvate or citric acid cycle intermediate C atoms are therefore called ‘glucogenic’. Those providing acetyl CoA or acetoacetate C atoms are called ‘ketogenic’. Some amino-acids are both glucogenic and ketogenic (their different C atoms flow to different places). 49 WHY DOES SKELETAL MUSCLE TRANSPORT N FROM CATABOLISED PROTEIN AS ALANINE? Recollect from Lecture 2 that other tissues transport the N as glutamine, but that muscle, in addition, can transport N as alanine. most body tissues working skeletal muscle body protein 1 amino-acids 2 2 Glu Glu 3 + 8 NH4 alanine 4 glutamine transported through plasma to liver 50 Step 8 in the scheme above is the transamination: Glu N + KG + pyruvate alanine N We traced the flow of the N (eventually to urea). But the alanine also carries C from the skeletal muscle to the liver. The C comes pyruvate. from skeletal muscle [see reaction above] In the liver, the transported alanine is transaminated back to Glu and pyruvate. [Step 9 in Lecture 2] 51 Thus, pyruvate, generated by glycolysis in working skeletal muscle, is transported to liver (in the form of alanine), where it can be used in gluconeogenesis. This process is similar to, and backs up, the Cori cycle, in which lactate is moved from working skeletal muscle to the liver. [Energy Transformations - Carbohydrates Lecture 4] Thus alanine transport simultaneously provides safe movement of N from catabolised muscle protein, and movement of pyruvate C for regeneration of Glc. 52 REMOVAL OF GLUTAMINE BY THE KIDNEY We have seen that N travels from many tissues through plasma as glutamine, and that the N of the glutamine, in liver, eventually becomes N of urea. Glutamine is also taken up by kidney, where, through glutaminase and glutamate dehydrogenase activity (Steps 5 and 7 of the scheme in Lecture 2], it is converted to 2 +NH4 and KG. This usually minor process increases during metabolic acidosis. [Acid-Base Balance lectures to come] Removal of protons as +NH4 in the urine helps increase plasma pH, as does KG metabolism through the citric acid cycle, because it generates CO2 (bicarbonate). 53 PHENYLKETONURIA Normally, phenylalanine (Phe) (one of the 20 coded amino-acids), is hydroxylated to tyrosine (another of the 20). [See R groups in Proteins Lecture 2] A genetic defect in the enzyme responsible leads to increased [Phe and its metabolites] in plasma and urine. Untreated, this leads to mental retardation, perhaps because the high [Phe] competes with movement of other aminoacids across the blood-brain barrier. Neonates are routinely screened for plasma Phe. 54 Treatment: restriction of dietary Phe (it is one of the ‘essential’ amino-acids) [Lecture 1] to amounts required for protein synthesis. Foods containing the artificial sweetener aspartame, which contains a Phe derivative, carry a health warning for those on a Phe-controlled diet.