Protein Metabolism Flashcards
transamination
general aspects —
1) utilized to generate energy
2) used for the synthesis of glucose
3) diverted for the formation of fat or ketone bodies
4) involved in the production of non essential amino acids
transamination—
- transfer of an amino —nh2 group from an amino acid to a keto acid.
- catalyzed by a group of enzymes - transaminases (aminotransferases)
- initiating point for amino acid metabolism
salient features
- all transaminases require pyridoxal phosphate (PLP) coenzyme derived from b6
- specific transaminases exists for each pair of amino and keto acids. however only two - aspartate transaminase (ast) and alanine transaminase - make a significant contribution for transamination
- no free nh3 liberated. only transfer occurs
- reversible
- very imp for redistribution of amino groups and prod of non essential aa. involves both catabolism and anabolism of aa
- diverts excess aa towards energy generation
- undergo transamination to finally concentrate nitrogen in glutamate. only aa that undergoes oxidative deamination to a significant degree to liberate free nh3, for urea synthesis
- all except lysine, threonine, proline and hydroxypoline.
- not restricted to alpha amino groups. eg, delta amino group of ornithine
- serum transaminases are imp for diagnostic and prognostic purposes
mechanism and significance of transamination
Mechanism
1) transfer of amino group to the coenzyme pyridoxal phosphate to form pyridoxamine phosphate
2) amino group of pyridoxamine phosphate is then transferred to a keto acid to produce a new aa and the enzyme with plp is regenerated
significance —
aspartate transaminase ast and alanine transaminase alt are very imp as diagnostic enzymes.
serum ast is elevated in cardiac disorders (mi)
- serum alt is increased in liver diseases (viral hepatitis)
deamination
- removal of amino group as nh3
- results in liberation of ammonia for urea synthesis
- simultaneously the carbon skeleton of amino acids is converted to keto acids.
- may be either oxidative and non oxidative
- occur simultaneously often involving glutamate as central molecule
I. oxidative deamination
- liberation of free ammonia from the amino group of amino acids coupled with oxidation
- takes place in liver and kidney
- purpose - to provide nh3 for urea synthesis and alpha keto acids for a variety of reactions, including energy generation
- role of glutamate dehydrogenase: in transamination amino groups are transferred to alpha keto acids to produce glutamate.
- thus glutamate serves as a collection centre for amino groups in the biological system
- unique - it can utilise either NAD+ or NADP+ as coenzyme
- conversion occurs formation of intermediate - alpha iminoglutarate
- reversibly links up glutamate metabolism with tca cycle
regulation of gdh activity - zinc containing mitochondrial enzyme
- complex structure - six identical rings;
- controlled by allosteric regulation
- gtp and atp inhibit - gdh
- gdp and adp - activate
- steroid and thyroid hormones inhibit
when ingestion of a protein rich meal liver glutamate level is elevated. converted into alpha ketoglutarate with liberation of nh3
- when cellular levels are low, degradation of glutamate is increased to provide alpha keto which enters tca cycle to liberate energy
oxidative deamination by amino acid oxidases:
L amino acid oxidase and D amino acid oxidase are flavoproteins possessing FMN and FAD etc.
- activity of LACO is low while that of DACO is high in tissues.
- LACO does not act on glycine and dicarboxylic acids.
fate -
- d aa are found in plants and microorganisms.
- regularly taken and metabolized by the body.
- daco converts them to respective alpha keto acids by oxidative deamination
- the alpha keto acids undergo transamination to convert to l aa
- keto acids may be oxidised to generate energy or serve as precursors for glucose and fat synthesis.
- thus d amino acid oxidase is important as it initiates the first step for the conversion of unnatural d amino acids to l amino acids
non oxidative deamination
some amino acids can be deaminated to liberate nh3 without undergoing oxidation
a) amino acid dehydrases: serine, threonine, and homoserine are the hydroxy aa
- they undergo non oxidative deamination catalyzed by plp dependent dehydrases
b) amino acid desulfhydrases: cysteine and homocysteine - desulfhydrases
c) deamination of histidine: enzyme histidase acts on histidine to liberate nh3 by a non oxidative deamination process
formation of ammonia
- from animo acids (transamination and deamination)
- biogenic amines
- amino group of purines and pyrimidines
- and by the action of intestinal bacteria (urease) on urea
transport and storage of nh3
- despite regular and constant production of nh3, it’s conc in circulation is surprisingly low (normal plasma - 15-45 ug/dl)
- mostly because - efficient mechanism for nh3 transport and immediate utilisation for urea synthesis
- between liver and tissues - form of glutamine or alanine and not as free ammonia
- alanine - muscle to liver
role of glutamine
- glutamine - storehouse of nh3
- highest conc in blood among aa (adults 8 mg /dl)
- synthesis - liver, brain and muscle
- ammonia removed from brain predominantly as glutamine
- freely diffusible in tissues and hence easily transported
- glutamine synthetase - responsible for synthesise of glutamine from glutamate and ammonia
- — unidirectional
- — requires atp and mg2+ ions
- deaminated by hydrolysis to release ammonia by glutaminase.
functions of ammonia and disposal
synthesis of many compounds
- non essential aa
- purines
- pyrimidines
- amino sugars
- asparagine etc
- ammonium ions are very imp to maintain acid-base balance of the body
disposal -
- ammoniotelic: aquatic animals - dispose nh3 into the surrounding water
- uricotelic : ammonia converted into uric acid eg reptiles and birds
- ureotelic : nh3 to urea. urea is non toxic and soluble compound hence easily excreted
toxicity of ammonia
even marginal elevation in the blood ammonia conc is harmful to the brain
- ammonia accumulates and results in slurring of speech and blurring if vision and causes tremors.
- may lead to coma or even death when left untreated
hyperammonemia: elevation may he acquired or genetic
- impairment in urea synthesis due to a defect in any one of the five enzymes involved in urea synthesis
- all these can lead to hyperammonemia and cause hepatic coma and mental retardation
- acquired may be due ti hepatitis, alcoholism etc where urea synthesis becomes defective
explanation for nh3 toxicity
- accumulation of nh3 shifts the equilibrium to the right with more glutamate formation hence more utilisation of alpha ketoglutarate
- alpha ketoglutarate is a key intermediate in tca cycle and its depleted levels impair the tca cycle
- net result- production of energy (atp) by the brain is reduced.
- toxic effects due to impairment in atp formation
trapping and elimination of ammonia:
- intravenous administration of sodium benzoate and phenyllactate is done
- these can respectively condensed with glycine and glutamate to form water soluble products that can be easily excreted.
- in some instances of toxic hyperammonemia - hemodialysis may be necessary
hepatic coma
- liver failure causes hyperammonemia - hepatic coma
- dysfunction of cns (convulsions) and impairment in liver function
- latter may result in hepatomegaly, jaundice, edema, haemorrhage etc
management
- restriction of protein diet
- use of antibiotics for bow disinfection
- proper maintenance of acid base balance
- avoiding use of hepatotoxic drugs
urea cycle
characteristics -
urea is the end product of protein metabolism
- urea accounts for 80-90% of the nitrogen containing substances excreted in urine
- urea is synthesized in liver and transported to kidneys for excretion in urine
- urea cycle first metabolic cycle that was elucidated by hans kreb and kurt henseleit hence it is known as krebs-henseleit cycle or ornithine cycle
- urea has two amino groups one from nh3 and other from aspartate
- urea synthesis is five step cyclic process with 5 enzymes
- the first two enzymes are present in mitochondria while rest in cytoplasm
steps : synthesis of carbamoyl phosphate
- carbamoyl phosphate synthase 1 cps1 catalyzes the condensation of nh4+ ions with co2 to form carbamoyl phosphate.
- step consumes 2 atp
- step is irreversible and rate limiting
- cps1 required n-acetylglutamate (NAG) for its activity (cofactor)
- another enzyme cps II
- – participates in pyrimidine synthesis
- – cytosomal enzyme
- – glutamine is the nitrogen donor
- – no cofactor needed
2) formation of citrulline
citrulline is syn from carbamoyl phosphate and ornithine by ornithine transcarbamoylase
- ornithine regenerated and reused
- ornithine and citrulline- basic aa
- citrulline produced is transported to the cytosol by a transporter system
3) synthesis of arginiosuccinate: arginiosuccinate synthase condenses citrulline with aspartate to produce arginiosuccinate.
- requires atp which is cleaved to amp and pyrophosphate (ppi) which is immediately broken down to inorganic phosphate (pi)
4) cleavage of arginiosuccinate: arginiosuccinase cleaves arginiosuccinate to give arginine and fumarate.
- arginine is immediate precursor for urea.
- fumarate liberated provides a connecting link with tca cycle.
5) formation of urea: arginase is the fifth and final enzyme that cleaves arginine to yield urea and ornithine.
- ornithine enters mitochondria through membrane transporter ornithine translocase for its reuse.
- hence ornithine may be considered a catalyst
- ornithine and lysine compete with arginine
- arginase is mostly found in liver.
overall reaction and energetics
- irreversible
- consumes 4 atp
- two atp are utilised for the synthesis of carbamoyl phosphate
- one atp is converted to amp and ppi to produce arginiosuccinate which equals to 2 atp.
- hence 4 atp are actually consumed
regulation of urea cycle
- the first reaction catalysed by carbamoyl phosphate synthase 1 (cps 1) is rate limiting reaction or committed step
- cps 1 is allosterically activated by nag
- the rate of urea synthesis is correlated with the concentration of nag (n-acetylglutamate)
- high concentrations of arginine increase nag
- the consumption of a protein rich meal increases the level of nag in liver leading to enhanced urea synthesis
- carbamoyl phosphate synthase I and glutamate dehydrogenase are localised in the mitochondria.
- they coordinate with each other in the formation of nh3 and it’s utilisation for the synthesis of carbamoyl phosphate
- the remaining four enzymes of urea cycle are mostly controlled by the conc of their respective substrate
disposal of urea
urea produced in liver freely diffuses and is transported in blood to kidneys and excreted
- a small amount of urea enters the intestines where it is broken down to co2 and nh3 by the bacterial enzyme urease.
- this ammonia is either lost in the feces or absorbed into the blood
- in renal failure the blood urea level is elevated (uremia) resulting in diffusion of more urea into intestine and its breakdown to nh3
- hyperammonemia (increased blood nh3) linked to uremia is commonly seen in patients of kidney failure
integration between urea and tca cycle
