Amino acid oxidation and the production of urea

1. Amino acid oxidation and the production of urea

2. Catabolism of proteins and aa nitrogen • How the nitrogen of aa is converted to urea and the rare disorders that accompany defects in urea biosynthesis • Normal condition- nitrogen intake match nitrogen excreted • Positive nitrogen balance- an excess of ingested over excreted nitrogen- during growth and pregnancy • Negative nitrogen balance – output exceeds intake- during surgery, advanced cancer or malnutrition

3. Oxidative degradation occur in 3 diff metabolic circumstances: • During normal synth and degradation of cellular protein- aa that are released from protein breakdown are not needed for new prot synthesis-degraded • Taking a protein rich diet- aa intake exceeds the body’s need for prot- degraded • During starvation or in uncontrolled DM – when carb cannot be utilized, proteins are used as fuel

4. Under all this metabolic conditions- aa lose their amino groups to form α-ketoacids (the carbon skeleton of amino acids) • The α-ketoacids undergo oxidation to CO2 and H20 and provide 3-4C for gluconeogenesis

5. Metabolic fates of amino groups • Metabolic fates carbon skeletons

6. Metabolic fates of amino groups • Degradation of ingested proteins to aa occurs in gastrointestinal tract • Entry of protein dietary will stimulate the hormone gastrin – will stimulate the production of HCl- kill most bacteria, denaturing agent for protein, unfolding globular proteins, and make internal peptide bonds more accessible to enzymatic hydrolysis • Pepsin is activated to cleave the polypeptide to smaller peptide

7. Dietary protein is enzymatically degraded to amino acids • Once protein are degraded to amino acids, aa are transported to liver- to remove the amino groups – by aminotransferases or transaminases • The amino groups are transferred α-ketoglutarate- forming glutamate

8. Glutamate releases its amino group as ammonia in the liver • Amino groups must be removed from glutamate for excretion • In hepatocytes, glutamate is transported from cytosol to mitoch – undergoes oxidative deamination- cat by glutamate dehydrogenase produced NH4+ • NH4+ is transported by glutamine to liver to be secreted thru urea cycle

9. Excretion of excess nitrogen • Excess nitrogen – excreted in one of three forms: ammonia (as ammonium ion), urea and uric acid • Fish – excrete ammonia – protected by the toxic activity through excretion and rapid dilution by environment • Terrestrial animals- excrete urea- water soluble compounds • Birds- excrete uric acid – insoluble in water- to avoid excess weight • High blood urea level as a consequence (not a cause) of impaired renal fx

10. Urea cycle • Central pathway in nitrogen metabolism- urea cycle • Start with the reaction of ammonium ion and C02 to produce carbamoyl phosphate • Step 1- Carbamoyl phosphate + ornithine →citrulline (carbamoyl phosphatase 1 • Citrulline + Nitrogen (2nd)→arginino succinate • Arginino succinate → fumarate and arginine • Arginine → urea and regenerate ornithin • Fumarate enter the TCA cycle

11. Ammonia intoxication • Ammonia produced by enteric bacteria and produced by tissues are rapidly cleared from circulation by the liver and converted to urea • Ammonia is toxic to central nerveous system • Ammonia levels may rise to toxic levels in impaired hepatic function – cirrhosis • Ammonia is toxic to brain – reacts with α-ketoglutarate to form glutamate – depleted levels of α-ketoglutarate – impair fx of TCA cycle in neurons

12. Ammonia intoxication • Mammals with genetic defects in any enzyme involved in urea formation cannot tolerate protein rich diet- as free ammonia cant be converted to urea- lead to hyperammonemia • Protein free diet is not an option. Mammals are incapable of synthesizing all 20 amino acids, thus must come from diet

13. Amino acid catabolism • Involve transferring the amino nitrogen to α-ketoglutarate to produce glutamate- leaving behind the carbon skeleton • After removal of their amino groups, the carbon skeleton of aa undergo oxidation to compounds that can enter the TCA cycle • In liver

14. The fate of carbon skeleton

15. Amino acid catabolism

16. The fate of carbon skeleton • Glucogenic aa yields pyruvate or OAA on degradation -----> gluconeogenesis • Ketogenic aa breaks down to acetyl-CoA or acetoacetyl-CoA- leading to the formation of ketone bodies

17. Six aa are degraded to pyruvate • Alanine, tryptophan, cystein, serine, glycine, and threonine • All are converted to pyruvate • Pyruvate can either be converted to acetyl-CoA (Ketone bodies precursor) • Or converted to OAA (gluconeogenesis)

18. Glycine degradation • Can be degraded in 3 ways – only one yield pyruvate • Involve conversion of glycine to serine via catalysis of serine hydroxymethyltransferase. Serine is then converted to pyruvate

19. Glycine degradation 2) Glycine undergoes oxidative cleavage to CO2, NH4+ and a methylene group – catalysed by glycine cleavage enzyme (or glycine synthase) • This pathway is critical in mammals • Defects in glycine cleavage enzyme- lead to elevated serum of glycine – inhibit neurotransmitter- mental retardation

20. Glycine degradation 3) Conversion of glycine to glyoxylate by D-amino acid oxidase. Glyoxylate is further oxidized to oxalate • Crystals of calcium oxalate account for 75% of all kidney stones

21. Ketogenic amino acid • Phenylalanine, tyrosine, isoleucine, leucine, tryptophan, threonine, and lysine • Breakdown of trp – precursor to synthesis • NAD and NADP in animals • Serotonin – a neurotransmitter in vertebrate

22. Phenylalanine catabolism • Phenylalanine- precursor for dopamin (a neurotransmitter) and hormones (norepinephrine and epinephrine) • Breakdown of phenylalanine- catalysed by phenylalanine carboxylase • Deficiency of this enzyme lead to phenylketoneuria (PKU) disease – due to elevated levels of phenylalanine

23. PKU • Defiency of phenylalanine carboxylase cause phenylalanine undergoes transamination with pyruvate – yield phenylpyruvate – • Accumulate in blood and tissues- excreted thru urine • Other than being excreted as urine directly, some of phenylpyruvate are reduced to phenylacetate – give odor to urine – nurses have traditionally used to detect PKU in infants • Accumulation of phenylalanine in early life impairs normal development of the brain- mental retardation • Due to excess of phenylalanine competing with other aa for transport across the blood brain-barrier- deficit required metabolites • Mental retardation can be prevented by rigid diet- provide only enough phenylalanine

24. The fate of branched amino acid • Isoleucine, leucine, and valine • Degraded only in extrahepatic tissues – oxidized as fuels in muscles, adipose, kidney • Due to the presence of branched-chain α-ketoacid dehydrogenase complex- absent in liver

25. Maple syrup urine disease • Due to the defective branched-chain α-ketoaciddehydrogenase complex- • Lead to accumulation of leucine in blood- and excreted to urine – smell like maple syrup • Untreated lead to abnormal development of the brain, mental retardation, and death in early infacy • Treatment include limiting the intake of valine, isoleucine, and leucine

26. Metabolic disorders related to urea cycle • Defects in urea synthesis results in ammonia intoxication • More severe if the metabolic blocks occur at reactions 1 or 2 – some intermediate compound have been synthesized • Due to defects in any several enzyme involved in the cycle