Amino Acid Oxidation

Fates of Amino Acids
🠶 Amino Acid Utilization
🠶 Amino-group metabolism
🠶 Explain role of transamination reactions in aa synthesis and identify vitamin essential for this reaction (tie in to urea cycle)
🠶 Describe interconversion between ketoacids and AA, including requirement of
pyridoxal phosphate (PLP) as a cofactor
🠶 Outline formation and transport of ammonia
🠶 Describe importance of reactions catalyzed by glutamine synthetase, glutaminase, and glutamate dehydrogenase
🠶 Ammonia Intoxication
🠶List causes for hyperammonemia, its consequences, and treatments to reduce blood ammonia levels

1. Amino-acid Oxidation Bio-Chemistry Rahul SIR Lecturer, Department of Med-Surg Co-chair of the South East Asia Regional Hub within the Challenger’s Committee at Nursing Now Challenge, London, UK

2. Specific Learning Objectives 🠶 Fates of Amino Acids 🠶 Amino Acid Utilization 🠶 Amino-group metabolism 🠶 Ex p l a i n ro l e of tra n sam i nat i o n r e a ct i on s i n aa synt h e s i s and i dent i fy v i t a m i n essential for this reaction (tie in to urea cycle) 🠶 D escr i b e i nte r c o n ve r s i o n betwe e n ke t oa c id s and aa, i n c l ud i n g re qu i r em e n t of pyridoxal phosphate (PLP) as a cofactor 🠶 Outline formation and transport of ammonia 🠶 Describe importance of reactions catalyzed by glutamine synthetase, glutaminase, and glutamate dehydrogenase 🠶 Ammonia Intoxication 🠶 L i s t c a use s for h y p e ra m mon e m i a , i ts c o n s e q ue n ces, and trea t men t s t o re du c e blood ammonia levels

3. Overview of AA catabolism Fig18.1: Lehninger Principles of Biochemistry by David L Nelson, 6 th Ed

4. Amino Acid oxidative degradation It occurs in three metabolic circumstances: 1. During normal synthesis and degradation of Proteins (Protein turnover) • Some aa released from proteins breakdown and not needed for new protein synthesis 2. If diet is rich in protein and ingested aa exceeds the body needs for protein synthesis, in this case surplus catabolized 2. During starvation or in uncontrolled diabetes, when carbohydrates either unavailable or not properly utilized, in this case cellular proteins are used as a fuel

5. Co n t-- Under all above conditions, aa lose their amino groups to form α-keto acids and also form carbon skeletons of aa: • α-keto acids undergo oxidation to CO 2 and H 2 O • 3-4 carbon units its converted into glucose by gluconeogenesis, fuel for brain, skeletal muscel and other tissues  Four aa plays imp role in nitrogen metabolism: Glutamate, Glutamine, (both converted to α-ketoglutarate), Alanine (to pyruvate) and Aspartate (to oxaloacetate)

6. Fates of Amino Acids 🠶 For Protein synthesis 🠶 For synthesis of other nitrogen containing compounds (heme, creatine, purines, pyrimidines, choline, neurotransmitters) 🠶 For gluconeogenesis 🠶 Energy source from glucogenic aa and ketogenic aa Glucogenic aa: Give rise to a net production of pyruvate or TCA cycle intermediates, such as α-ketoglutarate , succinyl CoA, Fumarate and oxaloacetate, all of which are precursors to glucose via gluconeogenesis. Ex. Ala & Arg

7. C o n t -- Ketogenic aa : Lysine and leucine are only aa are ketogenic, give rise to acetyl-CoA or acetoacetyl-CoA, neither of which can bring about net glucose production Glucogenic and Ketogenic aa : Small group of aa comprised of Ile, Phe, Thr, Trp, and Tyr give rise to both glucose and fatty acid precursors and characterized as glucogenic and ketogenic

8. Steps for Amino group catabolism 🠶 In cytosol of liver cells, amino groups from most aa transferred to α- ketoglutarate to form glutamate, which enters mitochondria and gives up its amino group to form ammonia 🠶 Excess ammonia generated in most tissues converted to amide nitrogen of glutamine, which passes to liver, then into liver mitochondria 🠶 In skeletal muscle, excess amino groups are transferred to pyruvate to form alanine 🠶 Aspartate come into play in metabolic processes that occur once amino groups delivered to liver

9. Amino group catabolism Fig18.2 (a): Lehninger Principles of Biochemistry by David L Nelson, 6 th Ed

10. Amino Acid Utilization Degradation of an aa in two stages: a) Carbon skeleton , is then converted to pyruvate, acetyl CoA, or citric acid cycle intermediate, depending on its makeup, with resulting energy production or energy storage b) Amino nitrogen atom is removed and converted to ammonium ion, which ultimately excreted from body as urea.

11. Amino-group metabolism • α - a m i n o grou p i s n i t r oge n so u rce d u r i n g aa metabolism • N i t r o g e n i s re m ove d f r o m a a a s a am m o n ia , which needs to be detoxified to urea Thr e e steps i n vo lv e d i n f lo w o f n i t r o g e n from a a to urea: (1) Tra n sa m i n at i o n (a m i no grou p t r a n sferr e d to glutamate), (2) Ox i d a t i ve dea m ina t i o n o f g l u ta m at e ( r e m ov al of amino group), (3) Synthesis of Urea Fig 19.15. Lippincott’s Illustrated Reviews, Biochemistry, 6 th Ed

12. Transamination • α-NH2 group of one aa is transferred to a α-ketoacid resulting in formation of a new aa and a new ketoacid • Donor aa (I) becomes a new ketoacid (I) after losing the α-NH2 group, and recipient ketoacid (II) becomes a new aa (II) after receiving the NH2 group Text Book of Medical Biochemistry by Chatterjee & Rana Shinde, 8 th Ed

13. C on t -- • α-amino group from L-amino acid is transferred to α-carbon atom of α- ketoglutarate, produced α-keto acid and glutamate • Transfer of amino groups from one carbon skeleton to another is catalyzed by aminotransferases • All aminotransferases have prosthetic group, which is pyridoxal phosphate (PLP), coenzyme form of pyridoxine or vitamin B 6 Fig18.4: Lehninger Principles of Biochemistry by David L Nelson

14. C o n t-- • PLP participates in transfer of α-amino groups to α- ketoglutarate • Location: cytoplasm of all cells • Enzyme: Transaminases (aminotransferases) • Co-factor: Pyridoxal phosphate (PLP), derivative of vitamin B 6 • Co m mon d o n or/accepto r p a i r: α -keto g lu t arat e and glutamate

15. Cont-- 🠶 Glutamate function as excretion pathways that lead to elimination of nitrogenous waste products 🠶 A l l a a ex c ep t l y s in e an d th r eoni ne par t i c ipat e i n t r a nsa m i n a t i o n in their catabolism but they undergo deamination reaction 🠶 T w o a m i n ot r ansferas e reac t i o n s ar e cat a l yz e d b y a la n i n e aminotransferase (ALT ) and aspartate transferases (AST)

16. Cont-- Alanine aminotransferase: In this alanine is donor aa and α-ketoglutarate is recipient ketoacid resulting in formation of pyruvate and glutamate. • During aa catabolism, this enzyme functions in direction of glutamate synthesis. Aspartate aminotransferase: In this Aspartic acid is donor aa and α-ketoglutarate is recipient ketoacid. • D u r in g a a ca t ab o li s m, th i s e nzy m e t r ansfers groups from glutamate to oxaloacetate, a m i n o fo r m ing aspartate, which is used as a source of nitrogen in urea cycle. Fig 19.8. Lippincott’s Illustrated Reviews, Biochemistry, 6 th Ed

17. Diagnostic value of plasma aminotransferases conges t i v e h e a rt failure, li ver Alanine aminotransferase 🠶 Normal enzyme activity is 3 to 15 IU/L 🠶 It is entirely cytoplasmic 🠶 Increases in viral hepatitis, diabetes, damage Aspartate aminotransferase 🠶 Normal enzyme activity is 4 to 17 I.U/L 🠶 It is cytoplasmic and also mitochondrial 🠶 Increases in Liver diseases, muscular dystrophies, acute pancreatitis, leukaemias, acute haemolytic anaemia

18. Oxidative Deamination • G l utama te r e l e ase s i t s a m i n o g r ou p a s a m mo n i a in Liver • Amino groups from many of α-aa are collected in liver in form of amino group of L-glutamate molecules • Th e se a m in o gro u p s must ne xt b e rem o ved f r om glutamate to prepare them for excretion • In hepatocytes, glutamate is transported from cytosol into mitochondria, where it undergoes oxidative deamination catalyzed by L-glutamate dehydrogenase Fig18.7: Lehninger Principles of Biochemistry by David L Nelson, 6 th Ed L-Glutamate γ semialdehyde

19. Co n t -- 🠶 It is only enzyme that can use either NAD + or NADP + as acceptor of reducing equivalents 🠶 Oxidative deamination of glutamate is main mechanism for release of a a n i t r oge n a s am m on i a ( NH 4 + ) i n a revers i b l e react i o n . 🠶 Location : Mitochondria of hepatocytes 🠶 Allosteric regulation of oxidative deamination : High energy state inhibits GDH and low energy state stimulates enzyme.

20. Transdeamination 🠶 Transfer of amino nitrogen to α-ketoglutarate forms l-glutamate by glutamate aminotransferases 🠶 Hepatic l-glutamate dehydrogenase (GDH), which can use either NAD+ or NADP+, convert glutamate to α-ketoglutarate, releases this nitrogen as ammonia, this α-ketoglutarate used in TCA cycle and glucose synthesis 🠶 Conversion of α-amino nitrogen to ammonia by coordinated action of glutamate aminotransferase and GDH is “transdeamination”

21. Co n t -- 🠶 Liver GDH activity is allosterically inhibited by ATP, GTP, and NADH, and is activated by ADP 🠶 GDH rea c t i o n i s fre e l y reve r s i b l e , a nd a l s o funct i o n s i n aa biosynthesis 🠶 α-ketoglutarate formed from glutamate deamination can be used in citric acid cycle and for glucose synthesis

22. Reference Books 1) Text Book of Medical Biochemistry by Chatterjee & Rana Shinde, 8 th Ed 2) Biochemistry, Lippincott’s Illustrated Reviews, 6 th Ed 3) Harper’s Illustrated Biochemistry, 30 th Ed 4) Lehninger Principles of Biochemistry, 6 th Ed 23

23. T h a n k Y o u 