Protein Metabolism: Oxidation and Urea Flashcards
what are the 4 fates of dietary amino acids?
- protein synthesis
- energy production (CAC)
- biosynthesis
- urea excretion
what are the 3 divisions of protein oxidation?
- normal synthesis and degradation
- protein rich diet
- starvation or diabetes mellitus
draw out the 4 fates of dietary amino acids
dietary protein stimulates the release of ___ from gastric mucosa in the stomach. This then stimulates the release of ___ from parietal cells, and ___ from chief cells.
- gastrin
- HCl
- pepsinogen
what does pepsinogen/pepsin do?
- it is a protease
- hydrolyzes phenylalanine (Phe), tryptophan (Trp), and tyrosine (Tyr) polypeptide bonds
- **pepsinogen changes to it’s active form (pepsin) in the stomach due to low pH (1.0-2.5)
what does secretin do?
stimulates the release of bicarbonate from the pancreas
what does cholecystokinin do?
stimulates zymogen release from the pancreas
what are the 3 main pancreatic zymogens (proteases) released from the pancreas, and what are they precursors to?
- trypsinogen → trypsin
- chymotrypsinogen → chymotrypsin
- procarboxypeptidase A and B → carboxypeptidase A and B
- important to remember that zymogens are enzyme precursors that require a biochemical change (usually cleavage) to become active
where does amino group catabolism occur?
hepatocytes in the liver
what is the basic strategy of amino group catabolism in the liver?
separate the amine group, leave the carbon chain
what is the source of most amino acids?
diet
what are the 4 metabolically important amino acids for amino group catabolism? what do they do?
- glutamate
- glutamine
- aspartate
- alanine
- amine group carriers; precursors and common metabolites; entry and exit molecules from the CAC
describe the functions of glutamate, glutamine, and alanine in amino group catabolism
ammonia is toxic. what are some effects of too much ammonia?
- reduction in learning
- reduction in movement
- hyperacusis
- imbalance
- gait ataxia
- seizures
- loss of consciousness
- death
how does ammonia prevent GABA inhibition?
- crosses blood brain barrier and disrupts astrocyte K+ uptake
- high extracellular K+ will prevent GABA inhibition
- causes neuronal hyperactivity, seizures, oxidative stress, and death
how is ammonia toxicity controlled?
amine groups are stabilized as urea (vertebrates) or uric acid (birds and reptiles) for excretion
what are the 2 major amino acid catabolism reactions?
- transaminase reactions
- one-carbon transfers
describe transaminase reactions
- transfer amine groups
- PLP = pyroxidal phosphate
- common coenzyme
- carrier of amino groups
how is glutamate transported to the liver via glutamine transport in the blood?
- glutamine is transported to the liver
- glutamine is non-toxic
- most cells have glutamine synthetase
- common synthetic precursor
how it glutamine transported to the liver via the glucose-alanine cycle?
- cycle allows proteins to function as energy sources
- cycle occurs in anaerobic states
- G-A cycle and Cori cycle occur simultaneously
how is intracellular ammonia buffered?
by converting glutamate to glutamine
what does the urea cycle excrete?
liver nitrogen
where does the urea cycle occur
hepatocytes in the liver
describe the 4 major steps of the urea cycle
- forms urea, a stable amine-rich molecule that can safely excrete nitrogen
- 4 steps, 5 structural changes
- requires enzymes within the mitochondria and cytoplasm
- ATP-dependent
- ornithine is absolutely essential
what are the 2 feeder molecules for the urea cycle?
carbamoyl phosphate and aspartate
draw out the urea cycle beginning with carbamoyl phosphate and aspartate
remember, ornithine is super important
describe the percentages of NH3 that enter the liver, and the percentages of urea that is excreted as urine vs. ammonia excreted in fecal matter
what are the 2 levels of urea cycle regulation?
allosteric N-acetylglutamate regulation
- increased synthesis of:
- ornithine transcarbomoylase
- arginosuccinate synthetase
- arginosuccinase
- arginase
- allosteric carbamoyl phosphate synthetase I regulation
- responsible for production of carbamoyl phosphate
describe allosteric n-acetylglutamate regulation
describe how the citric acid cycle and urea cycle can be linked
- dependent upon mitochondrial transporters
- aspartate nitrogen donation is important
- amino acid synthesis
- urea cycle
what are the energy substrates produced by amino acid catabolism?
THESE ARE ALL REVERSIBLE
- glucogenic - amino acids that can be converted to glucose
- ketogenic - amino acids that can be converted to ketone bodies
- transaminase and one-carbon transfer reactions
- fuels 10-15% of cellular energy production
*don’t need to know which amino acids are keto- and gluco-genic
name the nonessential amino acids
- glutamate
- aspartate
- alanine
- asparagine
- serine
what are the conditionally essential amino acids?
- glutamine
- glycine
- arginine
- proline
- tyrosine
- cysteine
what are the essential amino acids?
- phenylalanine
- valine
- threonine
- tryptophan
- isoleucine
- methionine
- histidine
- leucine
- lysine
which 6 amino acids are degraded to pyruvate?
- tryptophan
- alanine
- cysteine
- serine
- glycine
- threonine
- they are degraded to pyruvate, which then feeds into the CAC or gluconeogenesis
which 7 amino acids are degraded to acetyl-coa?
- tryptophan
- lysine
- phenylalanine
- tyrosine
- leucine
- isoleucine
- threonine
- acetyl-coa feeds directly into the CAC
what are 3 important one-carbon transfer reaction cofactors?
- biotin
- s-adenosylmethionine
- tetrahydrofolate
- one-carbon group donors or recipients
- often methyl groups
- vitamins
which 5 amino acids are degraded to alpha-ketoglutarate?
- glutamate
- glutamine
- proline
- arginine
- histidine
- alpha-ketoglutarate is part of the CAC
- less exergonic because we don’t produce an NADH from the isocitrate -> alpha-ketoglutarate
- remember that whenever you have glutamate, you have alpha-ketoglutarate
which 4 amino acids are degraded to succinyl-coa?
- methionine
- isoleucine
- valine
- threonine
- succinyl-coa is part of the CAC
- less exergonic because we don’t produce NADH from prior 2 steps in CAC
which 2 amino acids are degraded to oxaloacetate?
- asparagine
- aspartate
- oxaloacetate is part of the CAC
- least exergonic because it it towards the end of the CAC, so we are not producing any of the NADHs, FADH2s, or GTPs, from previous steps
