Protein III & IV Flashcards
Explain amino acid degradation during the fed state
Insulin stimulates glucose utilization
Amino acids released in circulation after eating dietary protein travel via the hepatic portal system to the liver
In liver, amino acids are used to make proteins, mainly albumin
Amino acids that pass through liver enter amino acid pool and become available to other tissues for protein synthesis
Excess amino acids are deaminated and carbons skeletons are turned into intermediates of glycolytic pathway, TCA cycle, and lipid metabolism
Explain amino acid degradation during fasting state
Insulin secretion is low while glucagon secretion increases in response to low blood glucose
Liver responds with increased gluconeogenesis
Skeletal muscles respond to low blood glucose by providing substrates for liver gluconeogenesis such as amino acids and lactate
Amino acid catabolism begin with removal of amino groups and resulting carbon skeletons are converted into glucose or ketone bodies, which can be used for energy generation
What is important about alanine and glutamine?
Two most common non-toxic transport forms of ammonium ions
Explain alanine trafficking
Alanine traffics to the liver only where it is converted back to pyruvate by ALT and ammonium ions enter urea cycle
Explain glutamine trafficking
Glutamine traffics to the liver, kidney, and intestines
What happens to glutamine trafficked to kidney?
In kidney, glutaminase converts glutamine to glutamate and ammonium ions are released in the urine
Glutamate can then be converted to alpha-ketoglutarate and enter TCA cycle
What happens to glutamine trafficked to liver?
Glutamine converted to glutamate via glutaminase while ammonium ions enter urea cycle
What happens to glutamine trafficked to intestines?
Glutamine is converted into ammonium ions and glutamate by glutaminase
Glutamate can form alpha-ketoglutarate that can enter the TCA cycle
Glutamate can also be converted into ornithine, which then forms citrulline
Ammonium ions transported to liver where they enter urea cycle
Glucogenic Amino Acids
Majority of amino acids except lysine and leucine
Intermediate/Products - alpha-ketoglutarate, oxaloacetate, succinyl-CoA, fumarate, and pyruvate
Product - Glucose via liver gluconeogenesis
Ketogenic Amino Acids
Leucine and Lysine
Intermediates/Products - Acetyl-CoA and acetoacetate
Product - Ketone bodies
What is the function of glutamate dehydrogenase?
Major player in amino acids degradation during fasting/starvation
Acts mainly in the liver
Oxidizes glutamate to alpha-ketoglutarate, NADH, and ammonium
How is glutamate dehydrogenase regulated?
High levels of GTP and NADH inhibit the enzyme - high cellular energy charge
High levels of ADP activate the enzyme (low cell energy charge)
Most active under fasting or stress where requirement for energy generation through amino acid oxidation is needed
What do aminotransferases and mitochondrial glutamate dehydrogenase together do?
Act together to direct alpha-ketoacids towards energy yielding metabolism while funneling ammonia into urea cycle
Familial Hyperinsulinemic Hypoglycemia Type 6 (HHF6)
Cause - Mutation to GLUD1 gene that encodes GDH so GDH is insensitive to GTP inhibiton
Symptoms: Appear during high protein diet. Hyperammonemia due to elevated ammonia levels and hypoglycemia due to increased ATP which promotes insulin release
How is glycine degraded?
By the glycine cleavage system.
GCS is located in mitochondria and is reversible
Requires THF and N5,N10-methylene THF.
Glycine can be converted back to serine by SHMT (serine hydroxymethyltransferase)
Patient presents with elevated glycine, seizures, lethargy, and lack of muscle tone (hypotonia). What is the patient’s condition?
Glycine Encephalopathy
Caused by mutations in genes associated with glycine cleavage system.
Known as nonketotic hyperglycemia
How is propionyl-CoA degraded?
Propionyl-CoA is common intermediate in catabolism of isoleucine, threonine, methionine, and valine.
Propionyl carboxylase carboxylates propionyl-CoA to D-methymalonyl-CoA in reaction that requires ATP, CO2, and biotin as cofactor
Racemase isomerizes D-methylmalonyl-CoA to L-methylmalonyl-CoA
Mutase convertes L-methylmalonyl-CoA to succinyl-CoA and requires Vitamin B12 as cofactor
Propionic acidemia
Loss of propionyl-CoA carboxylase
Causes accumulation of propionic acid and metabolic acidosis
Symptoms include metabolic acidosis, vomiting, rapid breathing, and lethargy
Methylmalonic aciduria
Defect in mutase or deficiency in vitamin B12 results in accumulation of methylmalonate in serum and urine
What is important about the homocysteine-methionine cycle?
Contributes to methyl conjugation of drugs, hormones, neurotransmitters, DNA, RNA, and proteins
Allows homocysteine to be converted into methionine and cysteine
Reaction catalyzed by methionine synthase - requires Vitamin B12
What is important about methionine synthase?
Converts homocysteine to methionine.
Has absolute requirement for Vitamin B12
Only reaction in mammals that coverts N5-methyl THF back to THF
How is vitamin B12 absorbed?
Vitamin B12 is released from food in the stomach
In the ileum, the intrinsic factor binds vitamin B12 and helps its absorption
In the blood, transcobalamin transports vitamin B12 to tissues
What are the sources of Vitamin B12?
Bacteria are the only source. Vitamin B12 can be found in animal products from ruminants and shellfish
Vitamin B12 deficiencies
Actual B12 deficiency - lack of vitamin B12 in diet, usually with vegans
Functional B12 deficiency - Inability to absorb or metabolize vitamin B12
Pernicious Anemia - severe lack of intrinsic factor due to gastric atrophy , cause can be organ specific autoimmune. Presence of antibodies against the intrinsic factor present
What is the biochemical basis of Vitamin B12 deficiency?
Non-functional methionine synthase
Non-functional mutase reaction
What occurs with non-functional methionine synthase?
Causes trapping of THF as N5-methyl THF
Decrease in N5, N10 methylene THF -> Blockage in DNA replication
Folate can restore purine and thymidine synthesis, but cannot prevent homocysteine accumulation
What occurs with non-functional mutase reaction?
Accumulation of methylmalonate and malonate
Folate cannot prevent methylmalonate accumulation
What are the clinical features of Vitamin B12 deficiency?
Megaloblastic anemia - large, immature, homeglobin poor red blood cells because of impairment of DNA synthesis in bone marrow cells
Demyelination - damage to myelin sheath of neuron
Artherosclerosis - high homocysteine due to deficiency of Vitamin B12
Methylmalonic aciduria (methylmalonic acidemia)
Diagnostic metabolites: methylmalonyl-CoA, methylmalonate and homocysteine
How are branched chain amino acids degraded? What are the branched chain amino acids?
Leucine, Isoleucine, Valine
BCAAs bypass the liver and enter muscle where they are degraded by muscle branched-chain aminotransferase (BCAT) (requires PLP) - produces branched chain alpha-ketoacids
The branched chain ketoacids travel to the liver where they are oxidatively and irreversibly decarboxylated by branched chain alpha-ketoacid dehydrogenase (BCKDH) - produces branched chain acyl-CoAs
What product is formed from leucine degradation in the liver?
Acetyl-CoA and acetoacetate
What product is formed from isoleucine degradation in the liver?
Propionyl-CoA and acetyl-CoA (glucogenic and ketogenic)
What product is formed from degradation of valine in liver?
Propionyl-CoA (glucogenic)
Maple Syrup Urine Disease
Impaired activity of branched chain keto acid dehydrogenase (BCKDH), which results in accumulation of branched chain alpha-keto acids and increased serum levels of branched chain amino acids.
Symptoms include maple syrup odor to urine and brain edema.
Diagnostic metabolites - High BCAAs in blood and high BCKAs in urine
Inborn error in tyrosine degradation pathway. Patient presents with keratitis and photophobia.
Tyrosinemia II
Loss of tyrosine aminotransferase enzyme
Alkaptonuria
Inborn error in tyrosine degradation pathway.
Loss of homogentisate oxidase which results
Diagnostic metabolites - Homogentisate and alkaptone
Symptoms are black urine and joint destruction
Treat with nitisone which inhibits production of homogentisate
Inborn error in tyrosine degradation. Loss of fumarylacetoacetate hydrolase
Tyrosinemia I
Diagnostic metabolites: succinylacetone
What are the sources of ammonium ions?
- Aminotransferase reactions
- Deamination (serine, cysteine, histidine, and threonine)
- Purine and pyrimidine metabolism
How are ammonium ions used in the body?
- Synthesis of glutamine
- Synthesis of urea
- Excretion in urine as NH4+
What are the signs of ammonia toxicity?
Neurological effects
Brain swelling
Lethargy, stupor, vomiting, and convulsions
What organ converts ammonium ions to urea?
Liver
Where did the two nitrogens in urea originate from?
- Ammonia
- Alpha-amino group of aspartate
What are the basic steps of the urea cycle?
- Carbamoyl Phosphate Synthetase Reaction in mitochondria
- Ornithine Transcarbamylase Reaction in mitochondria
- Argininosuccinate Synthetase reaction in cytoplasm
- Argininosuccine Lyase Reaction in cytoplasm
- Arginase Reaction in cytoplasm
What occurs during the first step of the urea cycle with Carbamoyl Phosphate Synthetase?
Rate limiting step of urea synthesis
Carbamoyl phosphate synthetase, activated by N-acetylglutamine, converts ammonium into carbamoyl phosphate
What occurs during step 2 of urea synthesis with ornithine transcaramylase?
Ornithine transcarbamylase converts carbamoyl phosphate and ornithine to citrulline
What occurs during step 3 of urea synthesis with argininosuccinate synthetase?
Citrulline and aspartate are converts to argininosuccinate through the enzyme argininiosuccinate synthetase
What occurs during step 4 of urea synthesis with the enzyme argininosuccinate lyase?
Argininosuccinate is converted to arginine and fumarate via argininosuccinate lyase
What occurs in the last step of urea synthesis with the enzyme arginase?
Arginine is converted to ornithine and urea via arginase
How do different tissues participate in the urea cycle?
Small intestine - epithelial cells produce citrulline from glutamine
Kidney - Once in kidney, citrulline is converted into arginine and arginine is released in the blood. Referred to as intestinal-renal axis.
Liver is only organ that can do entire urea cycle
What are early step urea cycle enzyme deficiencies?
Deficiency in either carbamoyl phosphate synthetase I or ornithine transcarbamoylase
Results in severe hyperammonemia and is lethal
What are late step urea cycle enzyme deficiencies?
Less toxic since intermediates are less toxic than ammonium
Examples:
Citrullinemia - Loss of argininosuccinate synthetase. Diagnostic metabolite is citrulline
Argininosuccinic aciduria - loss of argininosuccinate lyase. Diagnostic metabolite is argininosuccinate
Hyperargininemia - Loss of arginase. Diagnostic metabolite is arginine
What are the treatment options for early cycle enzyme defects of urea cycle disorders?
Protein restriction - very severe
Removal of ammonium ions independent of urea formation - form water soluble conjugates with glutamine and glycine
What are the treatment options for late cycle enzyme defects of urea cycle disorders?
Removal of ammonium ions independent of urea formation - form water soluble conjugates with glutamine and glycine
Dietary intervention - supplement diet with high doses of arginine
