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NUTR 302 > Biotin > Flashcards

Biotin Flashcards

1
Q

What vitamin is biotin?

A

B7

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2
Q

Biotin properties

A
  • sensitive to heat
  • solvent extraction, heat curing & food canning
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3
Q

Biotin structure and active sites

A

the ketone makes it reactive and 6he 2 nitrogen’s are the active parts involved in reactions

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4
Q

What are the different forms of biotin?

A

8 stereoisomers possible
* only D-biotin found in nature & biologically active
* Biocytin = bound to lysine
* Carboxybiotin = active form

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5
Q

What is the active form of biotin?

A

Carboxybiotin

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6
Q

How does biotin function

A
  • Functions as the prosthetic group in 4 carboxylases, serving as a CO2 carrier & donor
  • Also some non-coenzyme roles
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7
Q

What 4 carboxylases does biotin serve as a prosthetic group?

A
  • pyruvate carboxylase (PC) in mito
  • propionyl CoA carboxylase (PCC) in mito
  • acetyl CoA carboxylase (ACC) in cytosol
  • 3-methylcrotonyl CoA carboxylase (MCC) in mito
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8
Q

How does biotin activate the enzymes

A
  1. biotin activation: biotin + ATP→ biotin-AMP (biotinyladenylate) + PPi
  2. biotin-AMP is added to an apocarboxylase (containging lysine) through covalent bond via holocarboxylase synthetase to become the holocarboxylase (biotin-enzyme) and releasing the AMP
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9
Q

How does the biotin on the holocarboxylase accept CO2 to be transferred?

A

Addition of CO2 to biotin requires ATP, bicarbonate and Mg2+, therefore the reaction requires energy
1. hydrolyze ATP to dehydrate HCO3 to a phosphorylated CO2
2. The biotinyl-enzyme can then be carboxylated with the release of the phosphate group

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10
Q

How does the biotin donate the CO2?

A

carboxylates the substrate
* requires Mg2+
* requires energy: ATP→ADP

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11
Q

Describe the PC enzyme

A

pyruvate carboxylase which is neccessary for gluconeogenesis
* converts pyruvate (from AA) → oxaloacetate (which then becomes PEP or goes to TCA)

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12
Q

What happens with PC deficiency?

A

would get build up of pyruvate which leads to lactic acidosis and inrease risk for ammonia

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13
Q

Describe the PCC enzyme

A

propionyl CoA Carboxylase is needed for odd chain FA metabolism (also Val, Ile and propionate)
* converts propionyl CoA → methylmalonyl CoA
* methylmalonyl CoA is then converted to succinyl CoA via a B12-dependant isomerase which can enter TCA

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14
Q

What happens with PCC deficiency?

A
  • propionic acidosis
  • ketoacidosis
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15
Q

Describe the ACC enzyme

A

acetyl CoA carboxylase needed for fatty acid synthesis
* converts acetyl CoA → malonyl CoA which eventually goes to fatty acids
* in cytosol = rate- limiting step for FA synthesis
* in mitochondria = regulation of FA uptake for oxidation

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16
Q

Deficiency in ACC

A

No inherited genetic deficiency of ACC documented because it is so important it would be lethal if it stopped working at any point

17
Q

Dsecribe the MCC enzyme

A

3-methyl-crotonyl-CoA carboxylase is needed for Leu catabolism
* leu becomes 3-methyl-crotonyl-CoA and MCC then carboxylates it to 3-methyl-glutaconyl CoA

18
Q

Deficiency in MCC

A

severe acidosis

19
Q

Biotin non-coenzyme functions

A
  • pharmocological roles
  • role in epigenetics
20
Q

pharmocological roles of biotin

A

Pharmacological levels of biotin acts to induce glucokinase
* promotes transcription & translation of gene
* liver = permits rapid uptake & metabolism of glucose
* pancreas = acts as the signal for insulin release

21
Q

Biotin role in epigenetics

A
  • Histone biotinylation via lys = down regulates certain genes
  • regulates transcription & cellular responses to DNA damage
22
Q

Biotin Metabolism

A
  1. intesinal lumen: protein complex broken down by GI proteases yielding biocytin which is broken down biotinidase to yield biotion
  2. into enterocyte: Biotin enters enterocyte cell through a multivitamin transporter, some biocytin enters enterocyte through diffusion(?)
  3. absorption: Secreted out as free biotin, some bound to proteins, and biocytin (which gets converted to free biotin by biotinidase in circulation)
  4. Liver: Liver extracts majority & is major site of biotin utilization & metabolism
  5. Some biotin is secreted from liver and goes to other tissue
23
Q

How is biotin absorption regulated?

A

absorption ↑ during deficiency and ↓ during excess by changing the number of functional carriers

24
Q

How can biotin be reused?

A

When protein is broken down during turnover it yields biocytin in the cell and through biotinidase it can be reused for intracellular biotin for carboxylase biotinylation or histone biotinylation

25
Q

Biotin reabsorption and excretion

A
  • renal reabsorption is essential for biotin homeostasis & conservation and uptake is carrier-mediated & Na-dependent
  • most excretion is as biotin, some other metabolites/degradation products
26
Q

Biotin food sources

A

Biotin is in almost all foods in relatively low content and is available as free biotin & biotinyl-proteins/ biocytion
* protein-bound = animal products, nuts, cereals
* free = vegetables, green plants, fruit, milk
* not commonly used in fortified foods

27
Q

Other sources of biotin

A

synthesized by microflora in large intestine
* not sure of availability
* urinary & fecal content of biotin > dietary intake

28
Q

what can effect biotin recycling

A

biotinidase impairment
* can lead to symptoms of deficiency

29
Q

Biotin DRIs

A

no RDA set so it is AIs (µg)
* M/F = 30 µg/d
* increases with lactation
* daily dietary intake is estimated at 50-300 μg/d
* TPN patients = 60 μg/day recommended
* no UL set

30
Q

What can be used to measure biotin status

A
  • blood/serum (insensitive since renal reabsorption will increase with less intake and serum can be normal even with clinical signs of deficiency)
  • urine for biotin and metabolites
  • 3-hydroxyisovalerate = measure of MCC activity
  • other
  • lymphocyte PCC activity
  • lymphocyte FA composition
31
Q

Inborn errors for biotin

A
  • Holocarboxylase synthetase deficiency
  • Biotinidase deficiency
32
Q

Holocarboxylase synthetase deficiency

A

multiple carboxylase deficiency which results in ↓ activity of all 4 biotin-containing carboxylases
* S/S = dermatitis, alopecia, severe ketoacidosis, seizures

33
Q

biotinidase deficiency

A

unable to release free biotin from foods & to recover it from enzymes = functional deficiency
* S/S = seizures, ataxia, developmental delay, skin rash & hair loss

34
Q

Biotin deficiency

A

Rare
* Activities of carboxylases ↓

35
Q

How do raw eggs effect biotin availability

A

Raw egg whites = contain avidin which binds biotin and make it unavailable but biotin is heat labile therefore cooking eggs denatures it & destroys biotin-binding property of avidin

36
Q

Why is at risk of biotin deficiency?

A
  • low levels in infants, alcoholics, & pregnancy
  • treatment with antibiotics can reduce microbiomes production
  • patients on TPN if not put in

NUTR 302 (20 decks)

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