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USMLE Biochemistry > Amino acids > Flashcards

Amino acids Flashcards

1
Q

What 2 amino acids are purely ketogenic?

A
  • Leucine

- Lysine

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2
Q

All amino acids except from one have both L and D configurations. What amino acid only has one configuration?

A

Glycine

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3
Q

What are the 2 main basic amino acids - have a positive charge?

A
  • Arginine (most basic)
  • Lysine
  • Also histidine, although only slightly so has no net charge at plasma pH
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4
Q

What 2 amino acids are found in histones, which give them a positive charge?

A
  • Arginine

- Lysine

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5
Q

What are the 2 main acdic amino acids, have a negative charge as they donate H+ ions making solutions more acidic?

A
  • Aspartate

- Glutamate

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6
Q

What type of amino acids are commonly found in the lipid bilayer of cell membranes?

A

Hydrophobic

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7
Q

What amino acids are swapped in sickle cell anemia?

A

Hydrophilic glutamate substituted for nonpolar/hydrophobic valine

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8
Q

What amino acid has a rigid structure and is found in collagen?

A

Proline

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9
Q

Are most amino acids glucogenic or ketogenic?

A
  • Glucogenic OR glucogenic and ketogenic
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10
Q

What can ketogenic amino acids not become?

A

Glucose

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11
Q

What are ketogenic amino acids converted into?

A

Acetyl CoA

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12
Q

What 2 amino acids are purely ketogenic?

A
  • Lysine
  • Leucine
    Both essential
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13
Q

What is the 1st step in phenylalanine metabolism?

A

Phenylalanine
-> Tyrosine

Enzyme: Phenylalanine Hydroxylase

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14
Q

What substance acts as a cofactor for phenylalanine hydroxylase?

A

Tetrahydrobiopterin (BH4)

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15
Q

What does Tetrahydrobiopterin lose and become when converting Phenylalanine into Tyrosine?

A

Dihydrobiopterin (BH2)

- Loses 2 Hs

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16
Q

What enzyme and molecule converts Dihydrobiopterin (BH2) back to Tetrahydrobiopterin (BH4)?

A

Dihydrobiopteridine Reductase

- NADH

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17
Q

What enzyme converts Phenylalanine

-> Tyrosine?

A

Phenylalanine hydroxylase

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18
Q

What is classic PKU due to?

A

Phenylalanine hydroxylase deficient

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19
Q

What other than defective Phenylalanine hydroxylase can cause PKU?

A

Defective / deficient BH4 cofactor

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20
Q

What is the most common inborn error of metabolism?

A

Penylketonuria

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21
Q

What are the signs/symptoms of PKU?

A
  • Musty smell in urine from phenylalanine metabolites
  • CNS symptoms - retardatio, sizures, tremor
  • Pale skin, fair hair, blue eyes
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22
Q

Why do those with PKU have pale skin, fair hair, blue eyes?

A

Lack of tyrosine conversion to melanin

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23
Q

How is pku mainly treated?

A

Dietary modifactions

  • Phenylalanine restricted - synthetic aa mixtures use for food
  • Phenylalanine level monitored
  • No aspartame
  • Need to consume tyrosine
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24
Q

Maternal PKU

A

Occurs in women w. PKU who consume phenylalanine

  • High levels act as a teratogen
  • Baby born w. microcephaly, congenital heart defects
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25
Q

When are babies screened for phenylalanine?

A

2-3 days after birth

- Maternal enzymes may normalise levels at birth

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26
Q

What are causes of defective BH4 (causes ~ 2% of PKU)

A
  • Defective dihydropteridine reductase

- Also impaired BH4 synthesis

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27
Q

What other hormones are decreases in BH4 (deficiency other than phenylalanine)?

A
  • Epinephrine, NE
  • Serotonin
  • Dopamine (thus increased prolactin)
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28
Q

How is BH4 deficiency treated?

A
  • Dietary restriction of phenylalanine
  • Supplementation of BH4
  • L-dopa, carbidopa -> dopamine
  • 5-hydroxytryptophan -> serotonin
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29
Q

What cofactor does tyrosine require?

A

BH4 (-> BH2)

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30
Q

What enzyme converts BH2 back to BH4?

A

Dihydropteridine Reductase

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31
Q

What molecule is used to convert BH2 back to BH4?

A

NADH (-> NAD+)

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32
Q

What enzyme does Metyrosine (a drug used for pheochromocytoma and hypertension) target?

A

Tyrosine Hydroxylase

- Tyrosine not converted to DOPA

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33
Q

What enzyme converts Tyrosine to Dihydroxyphenylalanine?

A

Tyrosine Hydroxylase

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34
Q

What enzyme converts DOPA to Dopamine?

A

DOPA decarboxylase (removes carboxylic acid)

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35
Q

What vitamin acts as a cofactor for DOPA decarboxylase?

A

B6

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36
Q

What inhibits DOPA decarboylase periphally ?

A

Carbidopa

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37
Q

What comination therpay can be used in patients in parkinsons?

A

Levodopa/carbidopa

  • Levodopa is a dopamine precursor
  • Carbidopa prevents peripherally conversion of L-DOPA to Dopamine - reducing side-effects
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38
Q

What enzyme converts Dopamine to NE?

A

Dopamine B-hydroxylase

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39
Q

What vitamin acts as a cofactor for Dopamine B-hydroxylase?

A

Vitamin C

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40
Q

What enzyme converts NE to Epinephrine?

A

Phenylethanolamine N-methyltransferase

- Adds methyl group

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41
Q

What acts as a cofactor for PNMT?

A

SAM (S-adenosyl Methionine)

- Donates methyl groups

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42
Q

What is SAM synthesised from?

A

ATP and methionine (aa)

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43
Q

What is the difference in structure between methionine and homocysteine (very similar structures)?

A

SAM (w/o methyl group and adenosine)

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44
Q

What molecules are created from SAM when it removes its methyl group in the conversion of NE to E?

A
  • Adenosine

- Homocysteine

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45
Q

How is methionine regenrated?

A

Regenerated from homocysteine and N5-MethylTHF

  • Makes Methionine (converted to SAM)
  • and THF
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46
Q

What vitamins are required to make methionine and therefore SAM?

A

Folate (converted to N5-Methyl THF)

- B12 acts as a cofactor for homocysteine to be converted to methionine (and N5-Methyl THF -> THF)

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47
Q

What is added to tyrosine (along with other steos to make Thyroxine)?

A

Iodine

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48
Q

What cells make melanin?

A

Melanocytes

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49
Q

What enzyme converts Tyrosine to DOPA quinone (which is then converted into melanin)?

A

Tyrosinase

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50
Q

What are the 2 main types of oculocutaneous albinism? and what causes them?

A
  • OCA Type 1 - Tyrosinase deficiency-

- OCA Type 2 - Tyrosine transporters

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51
Q

What are people with oculocutenous albinism at risk of?

A
  • Sunburns, skin cancer
52
Q

What is Chediak-Higashi Syndrome?

A
  • Immunodeficiency

- OCA type II: Transporter defect - MTs dysfunctional`

53
Q

What does ocular albinism result in?

A

Ocular albinism

54
Q

What is Tyrosine broken down into?

A

Homogentistic acid (HGA)

Then:

  • Fumarate and (glucogenic)
  • Acetoacetate (ketogenic)

By HGA Oxidase

55
Q

What enzyme breaks down HGA into Fumarate and Acetoacetate?

A

Homogentistic acid Oxidase

56
Q

How is alkaptonuria inherited?

A

AR

57
Q

What is alkaptonuria a deficiency of?

A

Homogentistic acid oxidase

58
Q

What molecule builds up in alkaptonuria and what does this result in?

A

Increased Homogentistic acid

  • Polymerisation -> dark pigment
  • Pigment deposited in connective tissue (ochronosis)
59
Q

What are the findings in alkaptonuria?

A
  • Dark urine when left standing (due to polymerisation), discolouration in infancy
  • Arthritis (large joints, knees, hips)
  • Black pigment in cartilage, joints
  • X-ray: calcification of intervertebral discs
  • Other symptoms later in life
60
Q

How is alkaptonuria diagnosed?

A
  • Elevated HGA in urine/plasma
61
Q

How is Alkaptonuria treated?

A

Dietary restriction

- Tyrosine an phenylalanine

62
Q

What 2 enzymes break down catecholamines?

A
  • Monoamine oxidase (MAO) Amine -> COOH

- Catechol-O-methyltransferase (COMT): Methyl to O2

63
Q

What are epinephrine and NE broken down into?

A 
Vanillylmadelic acid (VMA)
- Excreted in urine
64
Q

What is dopamine broken down into?

A 
Homovanillic acid (HMA)
- Excreted in urine
65
Q

How can pheochromocytoma be diagnosed?

A

Metanephrin and normetanephrine in 24 hour urine collection

OR 24 hr collection of VMAs in urine

66
Q

How does Selegline work in Parkinson’s?

A

MAO-b inhibitor

- Increase Dopamine

67
Q

How does Entacapone and tolcapone work in Parkinson’s?

A

COMT inhibitors

- Increase dopamine

68
Q

What are MAO inhibitors such as Tranylcypromine and Phenelzine used for clinically?

A

Depression (less used nowadays)

- Increase dopamine, NE and serotonin

69
Q

What is Tyramine found in and what can Tyramine-rich foods cause in those on MAOi medications?

A

Cheese, red wine, some meats ‘cheese effect’

Similar to dopamine

  • Sympathomimetic
  • Hypertensive crisis
70
Q

What amino acid is used to synthesise serotonin?

A

Tryptophan

71
Q

What is Tryptophan converted to first before being converted to serotonin (5-HT)?

A

5-Hydroxytryptophan

72
Q

What enzyme converts Tryptophan to 5-Hydroxytrptophan (precursor of serotonin)

A

Tryptophan Hydroxylase

- BH4 -> BH2

73
Q

What is required as a cofactor for tryptophan hydroxylase?

A

BH4 (-> BH2)

74
Q

What is carcinoid syndrome caused by?

A

GI tumours that secrete serotonin

75
Q

Normally how much of tryptophan is converted to serotonin?

A

1%

76
Q

how much of tryptophan is converted to serotonin in carcinoid syndrome?

A

Up to 70%

77
Q

What molecule is depleted in carcinoid syndrome and what disease does this cause?

A

Tryptophan

- Pellagra

78
Q

What are the effects of serotonin?

A
  • Diarrhea (serotonin stimulates GI activity)
  • Increased fibroblast growth and fibrogenesis -> valvular lesions
  • Flushing (other mediators also)
79
Q

What valves may be affected in patients with carcinoid syndrome? (due to increased serotonin)

A

Tricuspid and pulmonary

80
Q

What enzyme breaks down serotnin into 5-Hydroxyindole acetaldehyde?

A

MAO

81
Q

What is serotonin broken down into?

A

5-Hydroxyindole acetaldehyde (MAO)

-> 5-Hydroxyindoleacetic acid (5-HIAA)

82
Q

What will be found in the urine in carcinoid syndrome?

A

5-HIAA

83
Q

What are the symptoms of Pellagra?

A
  • Hyperpigmented rash
  • Exposed areas of the skin
  • Red tongue (glossitis)
  • Diarrhea and vomitting
  • CNS: dementia, encephalopathy
  • Dermatitis, diarhea and dementia
84
Q

What vitamin deficiency can cause pellagra?

A

Niacin

85
Q

What other than 5-HIAA can Serotonin be converted to?

A

Melatonin

86
Q

What acts as a cofactor in the conversion of Tryptophan to Niacin?

A

B6

87
Q

What is Niacin used to create?

A

NADH and NADPH

88
Q

What is Hartnup disease?

A

AR

  • Absence of transporter in PCT
  • Loss of tryptophan in urine
  • Symptoms of niacin deficiency
89
Q

What is Histidine converted to Histamine by and what is a cofactor and what also is produced?

A

Histidine Decarboxylase

  • Requires Decarboxylase
  • CO2 produced in reaction
90
Q

What is glycine (aa) required for?

A

Porphyrin ring - heme synthesis

91
Q

What are the Carbons and nitrogens in heme from?

A
  • Glycine

- Succinyl CoA

92
Q

What is arginine used to create?

A

Creatinine (muscles)

Urea (urea cycle)

93
Q

What amino acid and molecule does Nitric oxide synthase require?

A

Arginine and NADPH

To create Citrulline, Nitric oxide and NADP+

94
Q

What is glutmate in the NS?

A

Excitatory NTs

95
Q

What enzyme produces GABA from glutamate?

A

Glutamate Decarboxylase (B6 cofactor)

96
Q

What are the branched chain amino acids (valine, leucine, isoleucine) broken down by (enzyme)?

A

a-ketoacid dehydrogenase

97
Q

Where are branched chain amino acids (valine, leucine, isoleucine) broken down?

A

Muscle cells

98
Q

What complex is a-ketoacid dehydrogenase a part of?

A

Branched-chain a-ketoacid dehydrogenase complex

99
Q

What are the subunits in a-ketoacid dehydrogenase complex?

A
  • E1: a-ketoacid dehydrogenase
  • E2: dihydrolipoyl transacylase
  • E3: dihydrolipoamide dehydrogenase
100
Q

What are the cofactors for the a-ketoacid dehydrogenase complex?

A
  • Thiamine

- Lipoic acid

101
Q

What are the 3 steps of branched cahin amino acid metabolism?

A
  • Transamination
  • Decraboxylation
  • Dehydrogenation
102
Q

What is Valine broken down into?

A

Succinyl CoA (glucogenic)

103
Q

What is Isoleucine broken down into?

A 
Succinyl CoA (glucogenic)
Acetyl CoA (ketogenic)
104
Q

What is Leucine broken down into?

A

Acetyl-CoA
Acetoacetate
- Purely ketogenic aa

105
Q

What is maple syrup urine disease due to a deficiency in?

A

A-ketoacid dehydrogenase

106
Q

What will MSUD children have in their plasma?

A
  • Increased branched chain AAs

- And alpha-ketoacids in plasma

107
Q

What gives MSUD patients urine a sweet smell?

A

Alpha-ketoacid of isoleucine (all 3 branched AAs have alpha-ketoacids after transamination)

108
Q

What is the main problem in MSUD?

A

Neurotoxicity due to build up of leucine

109
Q

What will the clinical symptoms of MSUD be?

A

1st few days of life

  • Lethargy, irratibility
  • Apnea, seizures
  • Cerebral oedema
110
Q

How can MSUD be diagnosed>?

A

Elevated valine, leucine and isoleucine in plasma

111
Q

What is the treatment of MSUD?

A
  • Dietary restriction of branched-chain amino acids
  • Monitoring AA concentrations
  • Thiamine supplementation
112
Q

How is Homocysteine made?

A

Methionine and ATP create:
- SAM

SAM has adenosine and Methyl group removed to create:
- Homocysteine

113
Q

What is the enzyme that converts Homocysteine and Serine into Cystathionine?

A

Cystathionine Synthase (B6)

114
Q

What is cystathionine metabolised into?

A
  • a-ketobutyrate -> Succinyl-CoA

- Cysteine

115
Q

What does cystathionine synthase require as a cofactor?

A

B6

116
Q

What enzyme converts homocysteine back into Methionine?

A

Methionine synthase

117
Q

What does Methionine synthase require as a cofactor?

A

B12

118
Q

What is N5-methyl Tetrahydrogolate converted into by methionine synthase?

A

Tetrahydrofolate

119
Q

What is a normal homocysteine level?

A

5-15 micromoles/L

120
Q

What can cause mild-moderate elevations of homocysteine?

A
  • B12/folate/B6 deficiecny

- May be linked to increased risk of CV disease

121
Q

What is considered severe hyperhomocysteinemia?

A

> 100umoles / L

122
Q

What are the common symptoms of Homocystenuria?

A
  • Retardation
  • Lens discolouration
  • Long limbs, chest deformaties
  • Osteoperosis in childhood
  • Blood clots
  • Early atherosclerosis (stroke, MI)
123
Q

What is classic Homocystinuria due to?

A

Cystathionine B synthase (CBS) deficiency

124
Q

How is homocystinuria treated?

A
  • Avoid methionine
  • Increased cysteine (now essential aa)
  • B6 supplements
125
Q

What are other causes of Homocystinuria / elevations outside of CBS deficiency?

A
  • Methionine synthase deficiency

- Defective metabolism folate/B12 - MTHFR gene mutations

126
Q

How are kidney stones in those with cystinuria prevented?

A

Methionine free diet

USMLE Biochemistry (44 decks)

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