biotin Flashcards
(27 cards)
What is biotin also known as?
Vitamin B7
What is the primary function of biotin in metabolism?
Acts as a coenzyme for carboxylases involved in metabolism
-creation of biotin-enzyme complex
-CO2 carrier and donor
what 4 carboxylases does biotin play a role in? what is the purpose of these reactions?
1) pyruvate carboxylase (PC)
-conversion of pyruvate to OAA
2) propionyl coA carboxylase (PCC)
-conversion of propionyl coA to methylmalonyl coA
3) acetyl coA carboxylase (ACC)
-conversion of acetyl coA to malonyl coA
4) 3-methylcrotonyl CoA carboxylase (MCC)
-conversion of MCC to MGC
What are the chemical properties of biotin?
Sensitive to heat and exists in multiple forms
What is the only biologically active form of biotin in humans?
D-biotin
What is biocytin?
Biotin bound to the amino acid lysine
-inactive form
What is carboxybiotin?
The active coenzyme form that participates in metabolism
-bound to CO2 and needed for carboxylation
What are the two key steps in biotin activation? what enzyme is involved?
1) Biotin reacts with ATP → Biotin-AMP (biotinyladenylate) + PPi
2) Biotin-AMP transfers biotin to apocarboxylase containing lysine → Holocarboxylase (active enzyme) + AMP
-Holocarboxylase synthetase
what is the affect of pyruvate carboxylase deficiency?
lactic acidosis (build up of pyruvate is converted to lactate) and increased ammonia
PC essential for gluconeogensis and OAA regeneration for the TCA cycle
what is the affect of propionyl coA carboxylase deficiency? what other nutrient is needed?
propionic acidosis and ketoacidosis
-B12 needed for isomerization of methylmalonyl coA to succinyl coA
PCC involved with metabolism of odd-carbon FA
what is the affect of MCC deficiency?
severe acidosis
MMC essential for leucine breakdown to generate acetoacetate (ketone body) and acetyl coA
What are the non-coenzyme roles of biotin?
-Gene regulation (Histone biotinylation & epigenetics)
-Glucose metabolism through glucokinase induction
What transporter is involved in biotin absorption?
Multivitamin transporter
explain the absorption of biotin. how is absorption regulated?
1) ingested as protein-biotin complex which is broken down by:
-GI proteases (to biotinyl peptides)
-Biotinases (release biotin)
2) absorption of biotin and storage in tissues
absorption is regulated by changing the # of functional carriers
how is biotin transported?
Transported as:
1) free biotin
2) biotin bound to proteins
how is biotin excreted?
liver is main site for biotin metabolism/excretion
-excreted as biotin
-uptake is carrier mediated and Na+ dependent
What are the main dietary sources of biotin?
Found in a relatively low content in all foods - availible in free and protein bound forms
Protein bound: Animal sources, nuts, cereals
Free form: vegetables, fruit, milk
Microflora may also be able to synthesize biotin
what is the RDA of biotin? what is the UL?
no RDA set, estimated intake of 50-100μg
no UL set
What protein in raw egg whites binds biotin? what does this do?
Avidin
-the body doesn’t have a good mechanism for extracting biotin from avidin, so bioavailibility is decreased
What populations are at risk for biotin deficiency?
- Infants * Pregnant women * Alcoholics * Long-term antibiotic therapy * Patients on Total Parenteral Nutrition (TPN) without biotin supplementation
what ways is biotin status determined?
-serum levels (not sensitive so status may not be properly reflected)
-lymphocyte PCC activity
-lymphocyte FA composition
What is holocarboxylase synthetase deficiency?
Impairment of activation of all four biotin-dependent carboxylases
What are the symptoms of holocarboxylase synthetase deficiency?
- Dermatitis * Alopecia * Ketoacidosis * Seizures
What is biotinidase deficiency?
Inability to release free biotin from protein-bound biotin or biocytin
-functional deficiency
