How are amino acids used in the fasting and well-fed states?
- Fasting state: used to supply energy via breakdown of proteins funneled into TCA cycle - Well-fed state: converted into glucose or FAs for storage of glycogen and triacylglycerols respectively
What happens to ammonia released during amino acid catabolism?
- Urea cycle, excreted
What is meant by glucogenic AAs? Which are these? What is meant by ketogenic AAs? Which are these? Which are both?
- Glucogenic = These AAs can be converted into glucose via TCA intermediates (alpha-kg, succ coa, fumarate, OAA) and pyruvate – anything that generates OAA, an intermediate in gluconeogenesis - Ketogenic = These AAs can be converted into acetyl-CoA and acetoacetyl-CoA, can never become glucose - Gluco/ketogenic AAs = TIPhe mnemomic = all T AAs (thr, tyr, trp), iso, phe - Ketogenic AAs = all L AAs - Glucogenic AAs = all others
- All AAs except for TIPhe and L AAs
- TIPhe = try, trp, tyr, iso and phe
- L starting AAs – leucine, lysine
Major source of amino acids during fasting?
- Striated muscle protein
What enzyme is involved in regulating AA catabolism? How does regulation occur?
- Primarily regulated by liver glutamate dehydrogenase (in mitochondria) - Regulation of glutamate DH is by cellular energy charge - High energy (high GTP, high NADH) = inhibition of enzyme - Low energy (high ADP) = activation of enzyme
Regulation of GDH
- High energy (high GTP, high NADH) = inhibition of enzyme = decreased formation of alpha-KG - Low energy (high ADP) = activation of enzyme = increased formation of alpha-KG
Describe familial hyperinsulinemic hypoglycemia type 6. Causes? Result?
- Mutations to glutamate dehydrogenase enzyme rendering it insensitive to inhibition by GTP. - As a result, increased AA catabolism occurs in environment that is high energy - Leads to elevated levels of ATP and hyperammonemia - Increased ATP in beta-cell promotes insulin releases, resulting in hypoglycemia - Less active urea cycle with more ammonia production = hyperammonemia
Which AAs are converted into pyruvate?
- Mnemonic: Some Good Children are Pyrates - Serine, Glycine, Cysteine, Alanine = Pyruvate
Describe the synthesis of pyruvate from AAs.
Describe glycine cleavage system
- Mechanism to degrade glycine and produce N5 N10 methylene THF or reverse to glycine
Which AAs can be used to synthesize Oxaloacetate?
- Every AAs used to generate pyruvate, then pyruvate into OAA via pyruvate carboxylase - In addition, aspartate and asparagine – Ox with a big ASP (ass)
Describe synthesis of OAA from AAs
Which AAs can be used to synthesize alpha-ketoglutarate?
- Mnemonic: Greg’s Hot Girlfriends Are Pregnant Ovals - Glutamine, Histidine, Glutamate, Arginine and Proline (and ornithine)
Describe synthesis of alpha-ketoglutarate from AAs
Which AAs can be used to synthesize succinyl-CoA? How?
- Mnemonic: VOMIT - Valine, odd-chain fatty acids, methionine, isoleucine, threonine - These are converted into propionyl-CoA, which undergoes intermediates to become succinyl-CoA
Describe synthesis of succinyl-CoA from AAs.
What are inborn errors of metabolism relating to synthesis of succinyl-CoA from AAs?
- AAs = VOMIT (valine, odd chain FAs, methionine, isoleucine, threonine) - 1.) deficiency in propionyl-CoA carboxylase resulting in propionic / propionyl acidemia - 2.) deficiency in racemase resulting in D-methylmalonyl aciduria - 3.) deficiency in mutase (or Vit B12, required as cofactor) resulting in methylmalonyl aciduria
What disorders is vitamin B12 deficiency linked to?
Source of vitamin B12.
- Meats and shellfish, with ultimate source as bacteria. No vitamin B12 in plants
Is vitamin B12 found in plants?
- Absolutely not, never, heck no!
Describe vitamin B12 absorption into body and storage
- R-binders in salivary enzymes bind B12 and pass it onto intrinsic factor, produced by stomach parietal cells - Absorbed in ileum through receptor mediated endocytosis - Binds to transcobalamin and is transported to tissues - Preferentially distributed to liver as vit-B12:transcobalamin complex and stored there. Kidney has some B12 stores
Two types of B12 deficiencies?
- Actual = dietary deficiency - Functional = lack ability to take up or metabolize B12
What is pernicious anemia?
- Lack of intrinsic factor secreted by parietal cells of stomach
Function of vitamin B12 – what reactions is it involved in?
- Only needed by two enzymes: 1.) methymalonyl-CoA mutase and 2.) methionine synthase - 1.) Converts L-methylmalony-CoA (intermediate in taking VOMIT AAs from propionyl-CoA) to succinyl-CoA - 2.) Converts N5 methyl THF to THF while converting homocysteine to methionine
What is purpose of vitamin B12 in THF metabolism and keeping functional folate pool?
- Serves to take the most reduced form of THF = N5 methyl THF back to THF form and in the process converts homocysteine to methionine - THF has no other functions in most reduced form, only when in oxidized forms does it have specific functions, including in DNA synthesis
What molecules accumulate with a vitamin B12 deficiency? Explain which reactions these are implicated in?
- 1.) D & L methylmalonyl-CoA (D and L methylmalonate) and 2.) N5-methyl THF - 1.) In process of converting VOMIT AAs to succinyl-CoA - 2.) In converting homocysteine and N5 methyl THF into methionine and THF respectively
Which populations can be at risk for vitamin B12 deficiency?
Biochemical basis for megaloblastic anemia – explain how B12 deficiency leads to functional folate deficiency
- Vitamin B12 deficiency results in deficiency in methionine synthase deficiency and therefore accumulation of N5-methyl THF and concomitant decreased in concentrations of more oxidized forms of THF, therefore Vitamin B12 leads to a functional folate deficiency - N5 N10 methylene is one of these more oxidized forms that would decrease. This is required for DNA synthesis (thymidine) - N10 formyl THF is required for purine synthesis, it is however decreased now too - Therefore lack of THF in oxidation states blocks DNA replication - Result is production of megaloblastic anemia – RBCs with large cytoplasm, but unable to divide
What are the two causes of megaloblastic anemia?
- B12 deficiency - Folate deficiency
Explain why B12 deficiency leads to neurological problems.
- Model for the neuropathology is unclear - Standard model: block in mutase reaction, causes accumulation of methlymalonyl-CoA, which is used instead of malonyl-CoA in FA synthase, causing accumulation of aberrant odd-chain and BCFA causes demyelination - New model: methionine synthase reaction is blocked, leads to decrease in methionine. Methionine is protective in some animal models with cobalamin (b12). Methionine administration is used successfully to treat demyelination in clinic
How to decipher folate or vitamin B12 or combined deficiency in clinical setting?
- Megaloblastic anemia can occur when deficiency in both and can be treated with folate in either cases. Folate given to B12 pt with allow sufficient oxidized THF to be available for DNA synthesis and resolving of megaloblastic anemia - Folate alone will not resolve the neurologic symptoms that go along with vit B12 deficient patients – demyelination leading to brain and nerve damage. Methylmalonyl acidemia seen in vitamin B12 deficient pt, not in folate deficient pt - Combined deficiency seen in chronic malnourished (eg. Alcoholic pts)
Methionine is essential. Why, if you can make it from homocysteine?
- No good dietary source of homocysteine
Synthesis of methionine
Homocysteine hypothesis for atherosclerosis.
- High homocysteine is risk factor for atherosclerosis - Epidemiological studies link B-vitamin (B6, B9, B12) with reduced homocysteine levels, but no evidence of supplement benefit in randomized controlled trials
What are branched-chain amino acids? Metabolism / Pathway of BCAA degradation? Products?
- BCAA = LIV mnemonic = leu, iso, val - Metabolism: muscle has high levels of BCAT (branched-chain AA aminotransferase), which forms branched-chain alpha-ketoacids that are released into blood. BCKDH (branched-chain alpha-ketoacid DH) complex decarboxylates these BCAA in liver - Valine (see picture) - Isoleucine produces acetyl-CoA and propionyl-CoA, it is ketogenic and glucogenic AA - Leucine produces acetyl-CoA and acetoacetate, it is ketogenic AA only
What is Maple Syrup urine disease? Causes? Symptoms? Result? Treatment?
- Genetic defect in BCKDH that leads to accumulation of BC alpha-ketoacids in body - Symptoms: poor feeding, vomiting, slow/irregular breathing, ketoacidosis, hypoglycemia and neurological dysfunction - High incidence in old order menonite - Result: fatal in one week unless treatment - Treatment: reduced BCAA in diet and monitoring of serum BCAA levels
Describe how Phe, Tyr are degraded
List disorders of phenylalanine and tyrosine degradation. Name enzymes.
- Phenylketonuria: deficiency in phenylalanine hydroxylase - Tyrosinemia-II: deficiency in tyrosine aminotransferase - Alcaptonuria: deficiency in homogentisate oxidase - Tyrosinemia-I: deficiency in fumarylacetoacetate hydrolase
What enzyme is defective in phenylketonuria? Symptoms?
- phenylalanine hydroxylase - Symptoms: intellectual disability, recurrent seizures, hypopigmentation, eczematous skin rashes
What enzyme is defective in tyrosinemia-II? Symptoms?
- tyrosine aminotransferase - Symptoms: keratitis, photophobia, skin lesions on palms and soles, intellectual disability
What enzyme is defective in alcaptonuria? Symptoms?
- homogentisate oxidase - Symptoms: urine black upon standing, black pigmentation of cartilage and collage, joint destruction and arthritis
What enzyme is defective in tyrosinemia-I? Symptoms? Treatment
- fumarylacetoacetate hydrolase - symptoms: severe condition affecting liver, kidney and peripheral nerves, fatal at young age if untreated (liver failure) - Dietary management and nitisinone (inhibits coversion of p-hydroxyphenylpyruvate to homogentisate)
What are the degradation products of Tryptophan?
- Acetoacetyl-CoA - Acetyl-CoA - Fumarate
The blood concentration of alanine and glutamine are higher than the concentration of other AAs. How do these AAs help rid the body of toxic ammonia?
- Alanine and glutamine are released in greatest quantity from muscle - BCAAs transfer their nitrogens to alpha-ketoacids (forming alanine and glutamine) and they travel to the liver where urea is formed
Sources of ammonium ions?
- Majority: aminotransferase rxns, deamination (ser, cys, his, thr) and deamidation (asn, glut) - Purine / pyrimidine metabolism - Bacteria in gut
Describe reactions of urea cycle. Where is each reaction taking place?
Which AAs are degraded to acetyl-CoA and fumarate?
- To acetyl-CoA: Leucine, Lysine, Phe, Tyr, Trp, Iso - To fumarate: Trp, Tyr, Phe
What are the sources of the amide and carbonyl groups of urea?
- Ammonium ion and aspartate
How is the urea cycle regulated? Explain
- Carbomyl phosphate synthetase I = rate-limiting step - Carbomyl phosphate synthetase I is activated by N-acetylglutamate synthetase, which is activated by arginine - High levels of arginine are indicative of elevated peripheral blood ammonia levels
What is the RLS in the urea cycle?
- Carbomyl phosphate synthetase I
Which urea cycle product is a TCA intermediate? How is the TCA cycle connected to the urea cycle? Why is this important?
- Fumarate production in urea cycle links urea cycle to TCA cycle in what is known as urea-TCA bicycle - Urea cycle requires ATP energy. Fumarate generates energy in TCA cycle to offset ATP requirements by TCA cycle - OAA can be converted into aspartate and feed into urea cycle
Describe intestinal-renal axis. Significance in infants?
- Small intestine absorbs glutamine and converts it into ornithine - Ornithine is released into circulation and can take its ammonia to liver where it is placed on urea - Also acted upon by OTCase and converted to citrulline which enters into kidney and is acted on by late urea cycle enzymes to be made into arginine - This axis is only established post-natally and is a method for synthesizing arginine. As a result, arginine is essential for infants, not for adults who have this system well established
What does urea cycle defects and hepatic failure lead to?
Causes of hyperammonemia?
- Urea cycle enzyme defects - Liver failure due to alcoholism, viral hepatitis etc.
Consequences of hyperammonemia?
- Ammonia is neurotoxin, leads to lethargy, stupor, vomiting, convulsions and death - Leads to brain swelling, possible via increased glutamine synthesis in astrocytes causing osmotic flux into astrocytes and swelling proceeds
Treatment of argininosuccinate lyase deficiency
- As a result of this deficiency -- argininosuccinicaciduria - Treat with arginine – supports continued citrulline synthesis and continuation of cycle. Argininosuccinate is water soluble and relatively non-toxic, gets eliminated from kidney as waste - Argininosuccinate carries both nitrogens
Treatment of argininosuccinate synthetase deficiency
- As a result of this deficiency – citrullinemia - Treat with arginine and phenylbutyrate supplements - Cirtruline can be cleared directly, but only carries single nitrogen
Strategies used to lower ammonia levels in individuals with urea cycle defects
Protein restriction (not elimination d/t essential AA req or supply AA via alpha-ketoacids)
Hemodialysis (removes excess ammonia)
Phenylbutyrate (metabolite binds glutamine in water soluble form, which is excreted by kidneys) and benzoate (metabolite binds glycine in water soluble form, which is excreted by kidneys)
Mannitol (promotes diuresis)