Unit 6 - Amino Acid Degradation and the Urea Cycle Flashcards
what are the most abundant AA in serum? in cells?
serum: alanine
cells: glutamate and glutamine
when is the AA gradient highest? (as in, what is most important for maintaining gradient?)
glutamate and glutamine (both are collecting sites for ammonia groups)
in a free AA pool of 100 g, how much are glutamate and glutamine, and how much are essential?
50% are glutamate and glutamine
10% are essential
what are essential AA? (PVT. TIM HALL)
phenylalamine valine threonine tryptophan isoleucine methionine histidine argininte (for growth) lysine leucine
what 2 AA get essential when other AA are missing?
cystine essential if methionine low
tyrosine essential if phenylalanine low
what is endogenous PRO associated with? how much is there?
skeleton and other supporting tissues, mostly collagen
-usually 6-7 kg
what is intracellular PRO like?
dynamic state with continual turnover
what are GI inputs to AA pool?
70-100 g dietary PRO
32-200 g endogenous PRO from sloshed off intestinal cells
how many AA transport systems are there? what are the types? where else are there?
5 separate transporters, with overlapping specificity (high redundancy)
- either Na+ or H+ symporters
- same transporters are in the kidney
what is Hartnup’s disease? symptoms? treatment? diagnosis?
defect in transport system for neutral and aromatic AA from gut and renal tubules (esp. phe and trp)
- have pellegra symptoms, so treat with niacin
- will have high levels of neutral and aromatic AA in urine and feces
what is cystinuria? symptoms? treatment? diagnosis?
defect in transport system for basic AA and cystine from gut and renal tubules
- since cys is insoluble, it forms crystals to cause UTI and kidney stones
- treat with fluids and penicillamine (makes cys soluble)
in a 70 kg man, how much PRO is turned over a day, and how many undergo oxidative degredation?
400 g turnover
50 g oxidative degredation and replaced
how can protein breakdown be measured?
histidine residues of muscle PRO actomyosin are methylated posttranslationally
-when broken down, 3-methyl histidine is liverated and excreted into urine to be measured
when does catabolism of AA take place and what does this mean?
metabolic breakdown of AA to urea and CO2 is continuous drain that is reduced during starvation, but never turned fof
- thus needs daily intake of A to replenish pools
- -40-50 g/day
- 12-20 g/day of N2 is excreted, mostly from urea
positive VS negative N2 balance
+ N2 losses are less than intake (growing children, recovering adults)
- N2 losses are more than intake (wasting, starvation); if prolonged, it is fatal if loss reaches 1/3rd total body PRO
what AA are plant PRO usually deficient in?
lysine
methionine
tryptophan
what are the 3 ways ammonia is removed?
- glutamate dehydrogenase
- NH4+ + NADPH + H+ + a-KG glu + NADP+ + H2O - glutamine synthase
- glu + ATP + NH4 gln + ADP + Pi - carbamoyl phosphate synthase I and II
- NH4+ + CO2 + 2 ATP –> carbamoyl phosphate + 2 ADP + Pi
- -I is in mitochondria for urea cycle
- -II is in cytoplasm for pyrimidine nt biosynthesis (doesn’t use NH4+)
what is the major carrier of NH4+ groups in the blood?
glutamine
what is the cofactor for all transaminations?
pyridoxal phosphate (vit B6)
in what reactions does pyridoxine function as a cofactor?
- transaminations (all of them)
- decarboxylations
- dehydrations of beta-hydroxyamino acids
- racemizations of alpha-AA
- removal of H2S from cysteine
what are the free aldehyde VS aminated forms of B6?
pyridoxal phosphate = free aldehyde form
pyridoxamine phosphate = aminated form
these are easily interconvertable, and all transaminases have common enzyme mechanism with pyridoxal phosphate
what 12 AA can be acted upon by pyridoxal phosphate transaminases?
remove alpha-AA groups
- ala
- arg
- asp
- asn
- cys
- ile
- leu
- lys
- phe
- tryp
- tyr
- val
what AA are acted upon by asp. transaminase?
asp
ala
leu
tyr
what AA can easily be made from pyruvate?
alanine (ala + a-KG pyruvate + glu)
- efficient way to send 3 C back into liver to be converted into glucose, while disposing of NH4+ group (from glutamate)
- so ala is one of most abundant AA in blood
what AA do most transaminases converge on?
glutamate
- represents convergence of pathways and method of collecting amino groups in one AA
- glu then serves as specific donor of amino groups for production of nitrogenous waste products
- this is why glu is one of most abundant AA in cells
where is glutamate dehydrogenase located and why? cofactors? allosteric regulators?
in mitochondria (where a-KG is, and NADH is quickly used for E)
- use either NADH or NADPH (normally high ratios of NADPH : NADP, and low NADH : NAD, so always cofactor available in either direction
- activated by A/GDP, inhibited by A/GTP
- -so lowering E charge of cell accelerates AA oxidation
where is glutamate mainly deaminated?
in the liver
- if it needs energy, it makes NADH for e- transport
- if it has E, it will make NADPH for biosynthetic reaction
what are AA oxidases?
both L and D occur in kidneys and liver
-enzymes use tightly bound flavins as cofactors to convert AA and water into corresponding a-keto acid and NH4+
what do dehydratases do? how does this work with serine? what do serine and threonine dehydratase use as cofactors?
directly deaminate
- ser –> dehydrate, then hydrate –> pyruvate + NH3
- both use pyridoxal phosphate cofactor
what does homocysteine desulfhydrase do? what does it use as a cofactor? what happens to the product
HC + H2O –> –> H2S + NH3 + alpha-ketobutyrate
- pyridoxal phosphate cofactor used to remove both NH4 + S
- a-kb –> PCoA –> mmCoA –B12–> SCoA
why does the body convert free NH4+ to urea?
- constant excretion of NH4+ in urine would cause drastic changes in blood pH
- urea is readily soluble (over 10 M) and easily excreted
- -carries 2 ammonia groups
- -non protonatable (10% solution pH 7.2)
- -low reactivity (inert)
what is the stoichiometry for one turn of the urea cycle?
CO2 + NH4+ + 3 ATP + asp + 2 H2O urea + 2 ATP + 1 AMP + PPi + fumarate + 2 Pi
what urea cycle enzymes are in the mitochondria VS cytosol?
mitochondria: carbamoylphosphate synthetase, ornithine transcarbamoylase (steps 1 + 2)
cytosol: argininosuccinate synthetase and lyase, arginase (steps 3 - 5)
how is urea cycle regulated? (PRO-free diet VS high-PRO diet)
PRO-free: urea excretion accounts for only 60% of total urinary nitrogen, compared to 80% of normal diet
-levels of urea cycle enzymes decrease
high-PRO: (also in starvation) gluconeogenesis from AA is high, so levels of urea cycle enzymes increase several fold (eventually subside if starving, b/c will use fat stores)
what is the most common genetic defect regarding the urea cycle?
OTC (ornithine transcarbamoylase) deficiency, b/c X-linked
for argininosuccinic aciduria…
- sign
- blood
- urine
- intelligence
- enzyme defect
- ammonia intoxication
- NH4 and argininosuccinic acid
- arginosuccinic acid
- normal
- argininosuccinase
for hyperammoniemia…
- sign
- blood
- urine
- intelligence
- enzyme defect
- ammonia intoxication
- high NH4+
- N/A
- normal intelligence
- carbamoyl phosphate synthase
for arginiemia…
- sign
- blood
- urine
- intelligence
- enzyme defect
- ammonia intoxication
- high NH4 and arginine
- arg, lys, orn
- normal
- arginase
for citrullinemia…
- sign
- blood
- urine
- intelligence
- enzyme defect
- ammonia intoxication
- high NH4+ and citrulline
- citrulline
- mental retardation
- arginosuccinate synthetase
for hyper-ornithinemia…
- sign
- blood
- urine
- intelligence
- enzyme defect
- ammonia intoxication
- high NH4+, orotate, and ornithine
- ornithine
- normal
- ornithine transcarbamylase
how is one treated if one has an inborn error of urea cycle metabolism?
low PRO diet supplemented with arginine or citrulline, since arginine becomes essential
- also give sodium benzoate and sodium phenylacetate to bind up gly and gln to be excreted
- ammonia levels are lowered to make more nonessential AA
