Biotin

Question 1

What vitamin is biotin?

Answer 1

B7

Question 2

Biotin properties

Answer 2

• sensitive to heat
• solvent extraction, heat curing & food canning

Question 3

Biotin structure and active sites

Answer 3

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

Question 4

What are the different forms of biotin?

Answer 4

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

Question 5

What is the active form of biotin?

Answer 5

Carboxybiotin

Question 6

How does biotin function

Answer 6

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

Question 7

What 4 carboxylases does biotin serve as a prosthetic group?

Answer 7

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

Question 8

How does biotin activate the enzymes

Answer 8

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

Question 9

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

Answer 9

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

Question 10

How does the biotin donate the CO2?

Answer 10

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

Question 11

Describe the PC enzyme

Answer 11

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

Question 12

What happens with PC deficiency?

Answer 12

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

Question 13

Describe the PCC enzyme

Answer 13

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

Question 14

What happens with PCC deficiency?

Answer 14

• propionic acidosis
• ketoacidosis

Question 15

Describe the ACC enzyme

Answer 15

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

Question 16

Deficiency in ACC

Answer 16

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

Question 17

Dsecribe the MCC enzyme

Answer 17

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

Question 18

Deficiency in MCC

Answer 18

severe acidosis

Question 19

Biotin non-coenzyme functions

Answer 19

• pharmocological roles
• role in epigenetics

Question 20

pharmocological roles of biotin

Answer 20

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

Question 21

Biotin role in epigenetics

Answer 21

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

Question 22

Biotin Metabolism

Answer 22

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

Question 23

How is biotin absorption regulated?

Answer 23

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

Question 24

How can biotin be reused?

Answer 24

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

Question 25

Biotin reabsorption and excretion

Answer 25

* 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

Question 26

Biotin food sources

Answer 26

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

Question 27

Other sources of biotin

Answer 27

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

Question 28

what can effect biotin recycling

Answer 28

biotinidase impairment * can lead to symptoms of deficiency

Question 29

Biotin DRIs

Answer 29

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

Question 30

What can be used to measure biotin status

Answer 30

* 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

Question 31

Inborn errors for biotin

Answer 31

* Holocarboxylase synthetase deficiency * Biotinidase deficiency

Question 32

Holocarboxylase synthetase deficiency

Answer 32

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

Question 33

biotinidase deficiency

Answer 33

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

Question 34

Biotin deficiency

Answer 34

Rare * Activities of carboxylases ↓

Question 35

How do raw eggs effect biotin availability

Answer 35

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

Question 36

Why is at risk of biotin deficiency?

Answer 36

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