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Biochemistry II > Amino metabolism - AA III > Flashcards

Amino metabolism - AA III Flashcards

1
Q

As a summary..

Which AAs are degraded to pyruvate?

A
  • Trp → Ala
  • Gly → Ser
  • Cys

REMEMBER: pyruvate → OXA

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2
Q

How can Ala be converted to pyruvate?

A

via ALAT = reversible transamination
α-KG + Ala → glu + pyruvate

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3
Q

What are the 2 pathways how Ser can be synthesized?

Where does it happen?

A

from

  • 3-P-glycerate (intermediate of glycolysis)
    → in cytoplasm
  • Gly by the action of Ser hydroxymethyl transferase
    → in mitochondrium

<u>NOTE:</u> SHMT catalyzes also reverse reaction, hence synthesis and degradation to another AA

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4
Q

How is Ser synthesized in the cytosol?

Where does it happen?

List its 3 steps.

A

synthesized from 3-P-glycerate in cytosol

  1. 3-P-glycerate → 3-P-pyruvate
  2. …. → 3-P-Ser
  3. …. → Ser
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5
Q

Which enzyme catalyzes the first step of synthesis of Ser from 3-P-glycerate?

Reaction + structures.

A

P-glycerate dehydrogenase

3-P-glycerate + NAD+ → 3-P-pyruvate + NADH

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6
Q

Which enzyme catalyzes the conversion of 3-P-pyruvate to eventually synthesize Ser?

Reaction + structures.

A

P-serine transaminase
transamination of 3-P-pyruvate, 3-P-Ser formed

3-P-pyruvate + Glu → 3-P-Ser + α-KG

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7
Q

Which enzyme catalyzes the conversion of 3-P-Ser to synthesize Ser?

Reaction + structures.

A

P-Ser phosphatase

3P-Ser + H2O → Ser + Pi

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8
Q

What are the 4 mechanisms of degradation of Ser?

Products.

A
  • to pyruvate
    • ​Ser-Thr ​dehydratase
    • Ser-pyruvate aminotransferase
  • to Cys: cystathione synthase/lyase:
  • to Gly: Ser hydroxymethyl transferase

<u>NOTE:</u> SHMT catalyzes also reverse reaction, hence synthesis and degradation to another AA

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9
Q

How can Gly be synthesized?

Enzyme + reaction.

Where does it happen?

A

Ser hydroxymethyl transferase = SHMT
this isoenzyme in cytosol

Ser + H4F ⇔ CH2-H4F + Gly + H2O

NOTE: can be considered part of catabolizing pathways of Ser, or anabolizing pathway of Gly

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10
Q

What is the physiological role of Gly?

A
  • inhibitory NT in NS
  • used for purine synthesis
  • conjugates, e.g. bile acids
  • heme biosynthesis
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11
Q

Where are the isoenzymes of SHMT located?

Function.

A
  • cytosolic SHMT: Ser → Gly
  • mitochondrial SHMT: Gly → Ser
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12
Q

As a summary..

What is the importance of SHMT?

A
  • synthesis of Gly from 3P-glycerate (through Ser)
  • synthesis of Ser from Gly
  • synthesis of methyl-group donor
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13
Q

What is the prosthetic group of SHTM?

A

PLP

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14
Q

How is the CH2-H4F that is necessary for the synthesis of Ser from Gly generated?

Which enzyme catalyzes this reaction?

A

Gly cleavage complex
degrades a second Gly

Gly + H4F + NAD+
→ HCO3- + NH4+ + CH2-H4F + NADH

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15
Q

What is the cause of nonketotic hyperglycinemia?

Consequences?

A

deficiency of Gly cleavage complex

→ mental deficiency, early death

Gly = inhibitory NT​

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16
Q

Ser is pretty important for cell membranes.

Why?

A

can be used to form

  • sphyngosine → incorporated into sphyngolipids
  • ethanolamine
  • choline
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17
Q

What is the biogenic amine of Ser?

Reaction + structures.

Function?

A

ethanolamine

Ser → ethanolamine + CO2

= second-most-abundant head group of phospholipids

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18
Q

As a summary…

Which AAs are converted to acetoacetate?

A

acetoacetate → acetyl CoA

  • Lys
  • Leu
  • Phe → Tyr
  • Trp
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19
Q

Which enzyme catalyzes the formation of choline?

Reaction + structures.

Function?

A

SAM dependent methylase

ethanolamine + 3 SAM → choline + 3 SAH

→ forms part of phospholipids

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20
Q

As a review..

How is Cys synthesized?

Which enzymes in its formation?

A

cystathione synthase/lyase
replace Ser-OH attached to HomoCys by -SH in 2 step reaction

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21
Q

What are the 2 degradational pathways of Cys?

What is the final product?

A

either degraded to

  • 3-mercapto pyruvate, or
  • sulfinyl pyruvate

→ eventually oxidized to pyruvate

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22
Q

Which enzymes are responsible for the formation of sulfinyl pyruvate?

Reaction steps + structures.

Which structures does it eventually form?

A

synthesized in 2 steps from Cys

  1. cysteine dioxygenase: Cys → Cys-sulfinate
  2. transamination:… → sulfinyl pyruvate

⇒ forms pyruvate + bisulfite

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23
Q

How is 3-mercapto pyruvate synthesized?

Which structures does it eventually form?

A

formed via transamination from Cys

⇒ -SH forms thiosulfite + pyruvate

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24
Q

How is sulfate formed?

Differentiate btw its 2 pathways.

A

originally derived from Cys

  • 3-mercapto pyruvate → thiosulfite
  • Cys sulfinate → sulfinyl-pyruvate → bisulfite

⇒ form sulfate

25
Q

What is PAPS?

How is it formed?
Reaction + structure.

A

= activated sulfate
formed in 2 steps

  1. sulfate + ATP → AMPS + PPi
  2. AMPS + ATP → PAPS + ADP + Pi

​⇒ most common coenzyme for sulfotransferase reactions

26
Q

How is taurine synthesized?

Reaction + structures.

Functions?

A

biogenic amine of Cys

  1. Cys sulfinate → CO2 + hypotaurine
  2. hypotaurine + 1/2 O2 → taurine

​⇒ conjugation of bile acids + cytoprotective function in brain

27
Q

As a summary…

Which AAs are degraded to α-KG?

A
  • Gln → Glu
  • His, Arg, Pro → Glu
28
Q

What are the common intermediates of synthesis AND degradation of Orn, Arg, and Pro?

A

form together w/ Gln Glu family
= all 3 AAs can be converted to Glu

  • Δ1-pyrroline-5-carboxylate
  • Glu-γ-semialdehyde

(spontaneous interconversion)

29
Q

As a review..

How is Arg degraded?

Structures.

A

in urea cycle conv. to Orn
→ Arg, Orn: same pathway of synth./degr.

  1. Orn transaminated to Glu-γ-semialdehyde
  2. dehydrogenated to Glu, NADPH
30
Q

How is proline degraded?

Enzymes?

A
  1. oxidized by POX → Δ1-pyrroline-5-carboxylate
  2. spontaneous conversion → Glu-γ-semialdehyde
  3. dehydrogenated → Glu, NADPH

​NOTE: dehydrogenation of Pro to its first product yields ROS

31
Q

What is the physiological role of Glu in the brain?

A
  • Glu = most important excitatory NT

can be used to synthesize its biogenic amine

  • GABA = most important inhibitory NT
32
Q

How is Glu produced in the brain?

A
  • from glucose + (mostly) BCAAs
    ​glucose conv. to α-KG, BCAAs then used for transamination
  • restored in glutamine cycle (cf. own card)
    after synpatic transmission
33
Q

What is the function of the glutamine cycle in the brain?

A

removes Glu from synapse after synaptic transmission, converted to Gln, shuttled back into neuron

  1. Glu enters cytosol of astrocyte
  2. conv. by cytosolic Gln synthetase
    Glu + ATP + NH4+ → Gln + ADP + Pi
  3. Gln released into extracellular space, taken up by neuron
  4. conv. by mitochondrial glutaminase
    Gln + H2O → Glu + NH4+
34
Q

What is the GABA shunt?

A

GABA degraded to Glu and succinate

→ shunt bc reaction mediated by α-KG DEH in citrate cycle (α-KG → succinyl CoA) bypassed

(only in presence of α-KG)

35
Q

Describe the mechanism of the GABA shunt.

Which enzymes are involved?

A

happening in 3 steps
last 2 steps mediate inactivation of GABA

  1. Glu decarboxylase
    Glu → CO2 + GABA
  2. GABA transaminase
    GABA + α-KG → Glu + succinate semiald.
  3. succinate semiald. DEH
    succinate semiald. + NAD+ + H2O → succinate + NADH
36
Q

What is the physiological relevance of Arg?

A
  • intermediate in urea cycle (forms Orn)
  • activator of Nac-Glu synthase
  • used for creatine synthesis
  • used for NO synthesis
37
Q

Describe the mechanism of creatine synthesis.

A

happens via intermediate guanidinoacetate (GAA)

  1. Arg + Gly → Orn + GAA
  2. SAM + GAA → SAH + creatine
38
Q

Describe the 2 step reaction of creatinine formation.

Enzyme + reaction.

A
  1. creatine kinase
    creatine + ATP → creatine-P + ADP
  2. dephosphorylated
    creatine-P → creatinine + Pi

⇒ then excreted via urine

39
Q

What is the physiological relevance of Orn?

A
  • intermediate in urea cycle
  • can used for synthesis of polyamines
40
Q

List the 3 polyamines.

What is their function?

A
  • putrescine
  • spermidine
  • spermine

​⇒ show amphipathic structure, able to interact w/ DNA and proteins to regulate cell cycle and apoptosis

41
Q

Which enzyme catalyzes the production of histamine?

Reaction.

What is the function of histamine?

A

= biogenic amine of His

His decarboxylase
His → CO2 + histamine

functions:

  • bronchoconstriction (via H1-R)
  • release of NO from endothelium (via H1-R)
    →     vasodilation
  • release of HCl from parietal cells (via H2-R)
42
Q

As a summary…

Which AAs are degraded to fumarate?

A
  • Phe → Tyr
  • Asp (aspartate cycle not relevant)
43
Q

Which enzyme catalyzes the synthesis of Tyr?

Reaction.

What is the consequence?

A

Phe hydroxylase

Phe + O2 + H4-biopterine
→ H2-biopterine + Tyr + H2O

NOTE: only Phe essential, since Tyr can be synthesized from it

44
Q

What is the product of catabolism of Tyr?

Name 1 important intermediate.

A

degraded to homogenisate → acetoacetate/fumarate

45
Q

What is the cause of PKU?

Consequence, treatment.

A

deficiency of Phe hydroxylase
Tyr becomes essential, Phe degraded to

  • phenyl-pyruvate
  • phenyl-lactate
  • phenyl-acetate

→ excreted via urine

⇒ ketoacidosis, mental retardation, can be treated w/ Phe free diet

46
Q

What is the physiological relevance of Tyr?

A
  • precursor of melanine
  • precursor of catecholamines (dopamine, nor-/epinephrine)
  • precursor of thyroid hormones T3, T4
47
Q

List the steps of the synthesis of catecholamines.

Where does it happen?

Differentiate btw the cellular locations of the different steps.

A

synthesized from Tyr in adrenergic postggl. axon terminals, adrenal medulla

  1. Tyr → dopa
  2. … → dopamine
  3. … → norepinephrine
  4. ... → epinephrine

ALL in cytosol, except step 3

48
Q

Which enzyme is responsible for the first step of the synthesis of catecholamines?

Reaction + structures.

Where does it happen?

A

Tyr hydroxylase
in cytosol

Tyr + O2 + H4-biopterine
→ dopa + H2O + H2-biopterine

basically same reaction as Phe hydroxylase

49
Q

What is the cofactor for the reaction catalyzed by Phe hydroxylase?

How does it regain its original structure once it participiated in the formation of Tyr?

A

H4-biopterine → H2-biopterine

⇒ dihydrobiopterine reductase
H2-biopterine + NADPH → H4-biopterine + NADP+

⇒ cofactor of Phe, Trp, Tyr hydroxylase

50
Q

Which enzyme catalyzes the conversion of DOPA in the formation of catecholamines?

Reaction + structures.

Where does it happen?

A

DOPA decarboxylase
in cytosol

DOPA → dopamine + CO2

51
Q

What is the prosthetic group of DOPA decarboxylase?

A

PLP

PLP = prost. group of all decarboxylases

52
Q

Which enzyme catalyzes the conversion of dopamine in the formation of catecholamines?

Reaction + structures.

Where does it happen?

A

dopamine β-hydroxylase
only reaction happening in vesicle, hence dopamine must be imported first

dopamine + O2 → norepinephrine + H2O

53
Q

Which enzyme is responsible for the fourth step of the synthesis of catecholamines?

Reaction + structures.

Where does it happen?

A

NE N-methly-transferase
(= SAM dep. methyltransferase)
in cytosol

norepinephrine + SAM → epinephrine + SAH

54
Q

What is the physiological relevance of Trp?

Can it be synthesized in the body?

A
  • precursor of NAD+
  • used for synthesis of serotonin, melatonin

​⇒ essential

55
Q

What are the products of degradation of Trp?

A
  • Ala
  • formate → formyl H4F (purine synthesis)
  • NAD+ precursor
  • further alternative pathways
56
Q

Describe the synthesis of serotonin.

Reaction + structures.

A

biogenic amine of Trp

  1. _Trp hydroxylase
    ​_Trp + O2 + H4-biopterine → 5-OH-Trp + H2O + H2-biopterine
  2. 5-OH Trp decarboxylase
    5-OH-Trp → CO2 + serotonin (5-HT)

<u>NOTE:</u> 1. reaction = similar to Phe/Tyr hydroxylase reaction

57
Q

As a summary..

Which AAs are non-essential, ergo can be synthesized?

A

10

  • Ala, Asp, Asn
  • Cys
  • Glu, Gln, Gly
  • Pro
  • Ser
  • Tyr

​AAA C GGG ProST

58
Q

As a summary..

What is the only semi-essential AA?

A

Arg

59
Q

As a summary..

List all essential AAs.

A

9/10

Biochemistry II (16 decks)

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