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Biochem > Amino Acid Metabolism > Flashcards

Amino Acid Metabolism Flashcards

1
Q

protein storage

A

no storage for proteins designated for energy metaboism
breakdown of body proteins only in times of great need -> the last resort

Cachexia; mm wasting, lipolysis

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2
Q

input to amino acid pool

A
  1. the intracellular synthesis of non-essential aminoacids
  2. tissue proteins
  3. dietary as source of essentail aminoacids
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3
Q

out puts of amino acid pool

A
  1. biomolecules w/ special functions; biogenic amines, thyroid hormones, heme, purine/pyrimidines, NO, etc
  2. degradation to the;
    - ammonia / urea
    - carbon skeleton; alpha-ketoacid -> pyruvated, acetyl CoA
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4
Q

C skeletons produced by aa degradation can be…(3options)

A
  1. oxidized to CO2 and H2O
  2. converted to glucose (glucogenic aa)
  3. converted to ketone bodies (ketogenic aa)
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5
Q

the NH3 produced by aa degradation can be (3 options)

A
  1. excreted in urine
  2. converted to urea then excreted
  3. used in synthesis rxns
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6
Q

first step of protein degradation

A

transamination - removal of alpha amino grp, freely reversible,
Aminotransferases (also syn non-essential aa from alpha-keto acid precursors)
PLP - B6

aa -> alpha-keto acid
alpha-ketoglutarate +NH2 grp -> glutamate

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7
Q

ALT & AST

A

ALT - alanine aminotransferase
AST - aspartate aminotransferase

liver is rich in both, AST can be just from mm check creatinine to confirm

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8
Q

second step of protein degradation

A

oxidative deamination - removal of aa group from glutamate

Glutamate Dehydrogenase (mito, liver, kidney)

glu -> alpha-ketoglutarate +NH3
special bc uses both;
NADP+ and NADPH depending on dir
forward rxn: +ADP, - GTP
reverse rxn: only when NH3 is high
anapleoritc rxn - another way to add intermediates to TCA cycle
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9
Q

transhydrogenation in mitochondria

A

ancillary rxn
NADH + NADP+ -> NAD+ + NADPH

mito need NADPH to reduced to keep glutathione in its functional form

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10
Q

transdeamination

A

combined process of transamination and oxidative deamination
most aa lose their alpha-NH2 this way
Thr & Lys are not substrates for PLP req aminotransferases -> req diff methods

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11
Q

transamination and the malate aspartate shuttle

A

in mito/cytosol OAA aspartate = transamination by AST

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12
Q

where does ammonia come from?

A

aa deamination
deamination of asn to asp, and gln-> glu
gut bacteria
etc

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13
Q

prob with ammonia

A

toxic to CNS. not sure why

perhaps due to dec in alpha-ketoglutarate

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14
Q

how is ammonia kept low;

A
  1. transport as glutamine (gln), alanine (ala)

2. conversion to urea by the urea cyle of the liver

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15
Q

why is it that making too much glutamine could be bad

A

rxn Glutamine synthetase requires ATP
decreases available glutamate which is imp for the neurotransmitter GABA
gln is osmoticly active -> cause edema in the brain

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16
Q

Glutamin synthetase

A

deals w ammonia problem;
glutamate -> glutamine (25% all circulating aa)
req ATP
brain, mm, liver

(reverse dir; glutaminase in liver and kidney)

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17
Q

most imp reason for low blood NH2 ?

A

rapid removal by the liver and conversion to urea

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18
Q

overall rxn of urea synthesis

A 
NH4 (imp source; ox deam of glu)
\+ HCO3(fr CO2) 
\+3ATP 
\+ NH2 (asp) 
-> urea
4 ATP ; 1 urea
5 rxns (2mito, 3cyto
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19
Q

carbamoyl phosphate synthetase 1

A

1st step of urea cycle; rate lim step
formation of carbamoyl phosphate
fr; CO2 and free ammonia
req N-acetylglutamate as an allosteric activator

*CPS 2 was in pyrimidine synthesis

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20
Q

ornithine transcarboamoylase (OTC)

A

2nd rxn of urea cycle
formation of citrulline from ornithine and carbamoyl phosphate
* only X-linked enzyme *

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21
Q

argininosuccinate synthetase (ASS)

A

3rd rxn of urea cycle
citrullin + asp -> arginosuccinate
driven by; hydrolysis of 3rd ATP
asp from transamination of glu by AST

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22
Q

lyase (ASL)

A

4th rxn of urea cycle;
cleavage of arginosuccinate to:
arginine (retains the N)
+ fumarate (carbon skel)

(recal purine ring synthesis)
fumarte = link to TCA

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23
Q

arginase

A

5th rxn of urea cycle
form orthinine + urea via cleavage of argnine

virtually exclusive to liver
ornithine re-enters urea cycle
urea diffuses into cycle

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24
Q

two amino acids you don’t find in protein. why?

A

citrullin and ornithine

no codons

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25
Q

why no urea in kidney? what happens there?

A

kidney makes arginine (arg) from citrulline
so, can generate Arginine here (thus arg is not really an essential aa)

lacks arginase => no urea

26
Q

liver and regulation of ammonia

A
  1. UC enzymes in periportal hepatocytes
    glutaminase & GDH (generate NH3) there as well -> produce ammonia where it will be cleaned up into urea quickly
  2. gln synthetase in perivenous hepatocytes (catch ammonia missed). Glu generated by reversal of GDH rx when NH3 is high

so little ammonia escapes into blood (due to this structure)
glutamine can escape, but thats ok

27
Q

what happens to the fumarate?

A

converted to malate via fumarase

malate - enters mito, malate dehydrogenase converts it to OAA generating NADH

28
Q

regulation of urea cycle

A

substrate conce
act of CPS 1 by N-acetyl glutamate made by NAG synthase
change in enzyme conc; long term increase 20-30 fold in starvation

29
Q

defect in ornithine transcarbamolyase

A

most common def in urea cycle
X-linked recessive
lethargy -> coma - > death ; encephalopathy

30
Q

tx of x-linked OTC

A
  1. severe protein restriction + supplementation w alpha-keto acid analogs of essental aa
  2. N-scavenging drugs; bind up non-essentail aa for excretion
  3. antibiotics - ammonia from bacteria
  4. supp w arg in some cases
31
Q

CPS 1 and CPS 2

A

both make carbamoyl phosphate
1 urea cycle, mito of liver, N from NH3, req NAG
2. de novo pyrimidine syn, cyto all nuc cells, N fr glutamine

32
Q

orotic aciduria

A

deficiency of OTC
due to increased syn of pyrimidine Orotic acid
OTC: CP + ornithine -> citrulline; OTC deficient, CP increaes and is used to make OA

33
Q

the three enzyems that fix NH3 into organic molecules;

A

Glutamate dehydrogenase
Glutamine synthetase
Carbamoyl phosphate synthetase 1

34
Q

2 aa solely ketogenic

A

leucine
lysine

C appear in aceytl/aceto acetyle CoA

35
Q

4(5) both keto and glucogenic

A 
tryptophan
isoleucine
tyrosine
phenylalanine
(threonine)
36
Q

solely glucogenic

A

alanine and rest of amino acids

metabolized to one of the following TCA intermediates:
pyruvate
OAA
fumarate
succinyl CoA
alpha-ketoglutarate

(then, converted to PEP, to glucose)

37
Q

the branched amino acids

A

leucine (leu)
isoleucine (ile)
valine (val)

taken up preferentially by skel mm
inc glucocorticoids and catecholamines -> aa released form mm -> skel mm initiates metabolism of BCAA (has the 1 aminotransferase)

38
Q

metabolism of BCAA generates

A

energy; ATP
glutamine
alanine
glucose

ala and gln also produced to carry ammonia

39
Q

branched chain amino acid aminotransferase

A

1 in muscle, plentiful here not in the liver

generates alpha keto acids from leu val ile

40
Q

branch chain alpha-keto acid dehydrogenase

A

muscle and liver
Coenzymes: TPP, CoA, lipoic acid, NAD, FAD)
like PDH and alpha-KGD, common E3, all generate NADH
also, like PDH, covalently reg; active if deP
inhibited by NADH

41
Q

Maple syrup urine disease

A 
definciency in BCKAD
accumulation of BCKA and BCAA
AR, mennonites
encephalopathy
missence E1
tx; dietary restriction of BCAA, balance thiamine tx, effective in rar
leu most toxic of BCAA
challenge; provid suff calories so don't force them into catabolism
42
Q

leu BCAA metabolism

A

acetoacetate + acetyl CoA
so leu; ketogenic
biotin dependent carboxylation required

43
Q

val BCAA metabolism

A

propionyl CoA -> succinyl CoA -> glucose

val: glucogenic

44
Q

ile BCAA metabolism

A

acetyl CoA and proprionyl CoA

so both ketogenic and glucogenic

45
Q

source of ala from muscle:

A

Muscle:
val/ile ->succinyl CoA -(TCA)-> OAA -PEPCK-> PEP ->
Pyruvate -(ALT)-> ala
glu -ALT-> alpha-KG

Liver:
ala transported to liver where glu and pyurvate form;
ALT; ala->pyruvate & alpha-KG -> glu

pyruvate can be used as substrate for gluconeogenesis

46
Q

glucose-alanin intertissue cycle

A

mucle produces pyruvate from glc & BCAA -> ala

liver: ala -(transamination)> pyruvate -(gluconeogenesis)> glucose

47
Q

source of gln sent out by mm

A

glu produced by transmaination rxn
BCAA -> BC alphaketo acids, while
alpha-KG -> glutamate

then glu -> gln via Glutamine sythetase

gln used by gut, liver, kidney

48
Q

what happens to gln in gut

A

glutaminase; gln -> glu + NH3

glu -> citrulline -> arginine (in kidney only)

or

glu + pyruvate -> alpha-KG + ala (via ALT)
ala -> glucose (in liver + kidney)

49
Q

what happens gln/glu in liver and kidney

A

glutaminase; gln -> glu + ammonia
GDH; glu -> alpha KG + ammonia
alpha-KG -> glucose

use of ammonia tissue dep; liver -> urea or gln

50
Q

ammonia produced in kidney

A

used in excretion of H+ in acidosis
especially imp in ketoacidosis

remember use of ammonia tissue dep

51
Q

which does the gut prefer KB or gln

A

KB, uses them of gln; sparing gln

gln goes to kidney and liver

52
Q

what amino acids have a special metabolism scheme?

A

aromatic aa:
trp
phe & tyr

53
Q

trp metabolism

A

an essential aa, keto&glucogenic

trp oxygenase, contains heme
ox cleavage of pyrrole ring = N-formylkurenine
hyrolytic removal -> kynurenine & formate
-> 3-hydroxykynurenine ->

3 Products

  1. alanine (glucogenic)
  2. quinolinate (-> NAD/NADP synthesis)
  3. acetoacetyl CoA (ketogenic)
54
Q

kenurenine can be

A
  1. catabolized to alanine & acetyle CoA (primary pathway)

2. met to quinolinate (minor, but phys sig pathway)

55
Q

quinolinate

A

from trp metabolism

quinolinate or niacin (frm diet) -> NAD & NADP

56
Q

Hartnup disorder

A

AR
defect in intestinal and renal abosorption of trp
symptoms; similar to niacin def (pellagra)
NT serotonin made from trp via hydroxylation (THB) and decarboxylation (PLP)

57
Q

metabolism of phe and tyr

A

phe & tyr - gluc and keto genic

Phenylalanine hydroxylase (liver)

  • Tetrahydrobiopterin (THB / BH2) gets ox
  • hydroxylates C4

phe (essential) -> tyr - Irreversible

complete def; Phenylketonuria (PKU)

58
Q

phenylketonuria (PKU)

A 
most common disease of enzyme deficiency
Phenylalanine hydroxylase (phe->tyr)
AR
phenylpyruvate (ketone) appears in urine
mousy musty odor
trp-> melanin (light skin/hair)
sev mental retaradation
Tx; diet low in phe + tyr supp
Phe = teratogen 
Hyperphenylalanemia -> could be a def in synth of BH4 too
59
Q

BH4 / THB

A

Tetrahydrobiopterin (THB / BH4)
involved in met of phe & tyr
phenylalanin hydroxylase
gets ox

def in this, or inab to reduce dihydrobiopterin back to tetra -> hyperphenylalaninemia

60
Q

degradation of tyr

A

noteworthy bc virtually every step has a known clinical conseq if enzyme is deficient;
focus on 2:
alcaptonuria
tyrosinemia 1

61
Q

alcaptonuria

A

trp degradation prob, AR = historical 1st

def; Homogentisate oxidase
homogentisate accumulates ox to dark pigment-like polymer -> urine, bone, CT

joint destruction, deterioration of cardiac valves

62
Q

glucogenic amino acids

A

all but leu and lys

Biochem (13 decks)

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