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CNS Biochemistry > Amino Acid Metabolism & Hyperamonemia > Flashcards

Amino Acid Metabolism & Hyperamonemia Flashcards

1
Q

Blood ammonia level is …..

A

5-35 umol/L

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2
Q

Mention sources of ammonia

A

Amino acids are quantitatively the most significant source of ammonia
1. From glutamine
2. From bacteria
3. From pyrines & pyrimidines
4. From amines either in diet or those which function as hormones & neurotransmiiters

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3
Q

Describe the mechanism by which glutamine produces ammonia

A

Glutamine is released from breaking down of branched chain amino acids in skeletal muscle, then produces ammonia in liver by action of glutaminase & glutamate dehydrogenase. In kidney, it is exreted as NH4+. In liver, it is detorified to urea.

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4
Q

Bacteria produce ammonia by …..

A

Urease

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5
Q

Describe phases of amino acid catabolism

A

1st phase, the removal of a-amino groups forming ammonia and a-keto acid
2nd phase, a-keto acid metabolised to energy-producing intermediates as glucose, ketone bodies, fatty acids, CO2 & water.

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6
Q

Define deamination

A

It is the removal of amino group from amino acid with formation of ammonia & a-keto acid.

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7
Q

Enumerate enzymes of oxidative deamination

A

Glutamate dehydrogenase
L-amino acid oxidases
D-amino acid oxidases

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8
Q

Describe Glutamate dehydrogenase reaction

A

Catalyzes reversible oxidative deamination of glutamate to form a-ketogltarate and ammonia thus functions in amino acid catabolism & synthesis
It requires NAD+ in oxidative deamination & NADP+ in reductive amination

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9
Q

Glutamate dehydrogenase is inhibited by….&activated by…..

A

ATP, GTP & NADH
ADP & GDP

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10
Q

D-amino acid oxidases deaminate …..& require ….coenzyme

A

Glycine
FAD

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11
Q

What is the importance of D-amino acid oxidases

A

The a-amino group removal removes the asymmetry of the compound thus a-keto acid can be aminated to L-amino acid used by the body.

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12
Q

Deaminases are …&….

A

Glutaminase & asparginase

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13
Q

The coenzyme of transaminases

A

PLP

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14
Q

Mention amino acids which do not undergo transamination

A

Lysine & threonine
Also cyclic imino acids proline & hydroxyproline

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15
Q

Define transdeamination

A

It is a coupling of transamination & deamination, transamination collects all the amino groups in L-glutamate which undergoes oxidative deamination in the liver releasing ammonia.

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16
Q

Classify amino acids according to fate

A
  1. Glucogenic amino acids form pyruvate, oxaloacetate or any intermediate of TCA that can be used in gluconeogensis
  2. Ketogenic amino acids form acetyl coA, acetoacetate or acetoacetyl coA and form ketone bodies, lysine & leucine
  3. Mixed a.a., can give both, tyrosine, tryptophan, threonine, phenylalanine, isoleucine
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17
Q

Mention fates of ammonia

A

Biosynthesis of urea, non-essential amino acids, monoamines, purines & pyrimidines
Small amounts are excreted in urine

18
Q

GR: Occurrence of ammonia toxicity

A
  1. Removal of excess ammonia requires reductive amination of a-ketoglutarate to glutamate then it is converted to glutamine by glutamine synthetase, this depletes a-ketoglutarate an intermediate of TCA & glutamate thus also GABA is depleted. Also glutamine formation depletes ATP
  2. High ammonia conc lead to inc permeability to K+ and Cl-
  3. Excess glutamine is exchanged for tryptophan a precursor of serotonin
  4. It also causes osmotic shift of water resulting in cerebral edema
19
Q

The first 2 steps of urea synthesis are in ….., while last 3 are in …..

A

Mitochondria
Cytosol

20
Q

Describe the energy consumption of urea cycle

A

3 ATP
4 high energy bonds

21
Q

Ammonia is transported in circulation as …&…

A

Glutamine & alanine

22
Q

Mention steps of urea biosynthesis

A
  1. Biosynthesis of carbamoyl phosphate
  2. Synthesis of citrulline
  3. Synthesis of argininosuccinate
  4. Cleavage of argininosuccinate
  5. Cleavage of arginine
23
Q

Define carbamoyl synthetase-1 reaction

A

Needs ammonia, CO2 ,Phosphate and energy and produces carbamoyl phosphatase consuming 2 ATP

24
Q

Ornithine transcarbomaylase forms…..

A

Citrulline

25
Q

Which enzyme of urea cycle converts ATP TO AMP

A

Argininosuccinate synthetase

26
Q

Describe reaction of angininosuccinate lyase

A

Cleaved into arginine and fumarate
Fumarate is used to regenerate aspartic acid

27
Q

Describe the reaction arginase

A

Arginine is cleaved into ornithine and urea

28
Q

Sources of urea nitrogen are…..

A

Ammonia and aspartate

29
Q

Describe short-term regulation of urea cycle

A

CPS1 is allosterically activated by N-acetylglutamate which is synthesized by N-acetylglutamate synthase enzyme allosterically activated by arginine.

30
Q

Describe link between TCA & Urea cycles

A

CO2 of urea c is formed by TCA
Ammonia is formed from glytamate by glutamate dehydrogenase
ATP is produced by TCA
Fumarate produced by urea cycle can be converted to oxaloacetate
Aspartic acid used in urea cycle is formed from oxaloacetate by transamination

31
Q

Describe causes of congenital hyperammonemia

A

Due to deficiency of,
Any enzyme of urea cycle
Membrane-associated ornithine transporter
N-acetyl glutamate synthase

32
Q

How does lactulose help in hepatic encephalopathy?

A
  1. Reduction of colonic bacteria overload
  2. Conversion of lactulose toblactic acid results in acidification of the gut tgus inhibiting ammoniagenic bacteria and inc lactobacilli, also increase pH gradient thus NH3 moves from blood to gut and NH3 of gut becomes NH4+ which can’t be absorbed.
33
Q

Describe role of phenylbutyrate in hepatic encephalopathy

A

Converted to phenylacetate which bind to glutamine forming phenylacetateglutamine which is excreted in urine thus ammonia is lost.

34
Q

Sodium benzoate action in hepatic encephalopathy

A

Interacts with glycine forming hippurate excreted in urine thus ammonia is lost.

35
Q

Explain role of Hepa-Merz in hepatic encephalopathy treatment

A

L-ornithine activates urea cycle
Also L-ornithine & L-aspartate are substrates for glutamate transaminase, thus they increase glutamate levels. And ammonia is tranferred to glutamate forming glutamine.

36
Q

Tyrosine is formed by enzyme ….. with …… as coenzyme

A

Phenylalanine hydroxylase
Tetrahydrobiopterin

37
Q

Describe clinical picture of phenylketuria

A

Mental retardation due to inability to form CA and accumulation of products in tissues. These toxic products toxic products are derived from abnormal metabolism of phenylalanine
Depigmentation due to inability to form melanin
Bad mousy odour of urine

38
Q

Treatment of phenylketuria

A

The earlier treatment starts the more completely neurologic damage is prevented, the goal of treatment is to keep plasma phenylalanine & its toxic products at low levels
Phenylalanine low diet
Supplement tyrosine & tetrahydrobiopterin
Enzyme substitute

39
Q

Maple syrup urine disease is deficiency of …., catalyzes ….step in catabolism of a.a.

A

Branched chain a-ketoacid dehydrogenase complex
Oxidative decarboxylation

40
Q

C/P of maple syrup urine disease

A

Mental retardation and may lead to early death
The urine has a sweet smell like burned sugar due to rise of isoleucine

41
Q

Treatment of MSUD

A

Diet low in branched chain amino acids

CNS Biochemistry (4 decks)

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