Riboflavin (B2)

Question 1

what are the coenzyme forms of riboflavin (B2)?

Answer 1

• flavin adenine dinucleotide (FAD)
• flavin mononucleotide (FMN)

Question 2

what are the functions of riboflavin coenzyme form?

Answer 2

• function as electron transfer reaction (2-step single-electron reactions)
• energy production in the TCA cycle, electron transport chain and fatty acid oxidation

Question 3

what is the role of free Riboflavin (B2)?

Answer 3

acts as a non-coenzyme form that is absorbed and transported throughout the body

DNA synthesis, antioxidant function (glutathione reductase)

Question 4

how is riboflavin transported in the tissue cells?

Answer 4

transported in its “free form” and converted to FAD/FMN in tissue cells via ATP-dependent reactions. The FAD form becomes free riboflavin in the lumen.

Question 5

what are the two enzymes that make up riboflavin (free form)?

Answer 5

flavin and ribitol

Question 6

what does FMN stand for?

Answer 6

flavin mononucleotide

Question 7

what does FAD stand for?

Answer 7

flavin adenine dinucleotide

Question 8

what are the sources of riboflavin?

Answer 8

milk/dairy, eggs, meats, spinach, legumes, mushrooms, and enriched cereals/breads

Question 9

what are the functions of riboflavin?

Answer 9

• coenzyme in flavoproteins
• energy transformation (ETC, oxidative decarboxylation of pyruvate and a-ketoglutarate, TCA cycle, and fatty acid oxidation)
• reducing enzymes (NADPH, FAD-dependent reductase)

Question 10

______ is the coenzyme used to combine with oxidized ________ into reduced glutathione (NADP+) + 2 GSH

Answer 10

FAD; glutathione

Question 11

what are the electron donors in glutathione reductase?

Answer 11

NADPH

Question 12

why is glutathione reductase essential?

Answer 12

maintains glutathione in reduced state (GSH)

Question 13

GSH is a key endogenous antioxidant for what (3) things?

Answer 13

1. maintaining SH group in proteins
2. maintaining heme in Fe2+ state
3. removal of toxic peroxides

Question 14

what is thioredoxin reductase (TrxR)?

Answer 14

flavoenzyme that reduces thioredoxin

Question 15

what is thioredoxin (Trx)?

Answer 15

proteins acting as antioxidants by facilitating the reduction of other proteins by cysteine thiol-disulfide exchange

Question 16

what are the purposes of thioredoxins?

Answer 16

• helps ribonucleotide reductase to make DNA
• electron donor (SH) to ribonucleotide
• used to regenerate S to SH

Question 17

how is riboflavin digested, absorbed and transported through the body?

Answer 17

• FAD/FMN (coenzyme) in foods are hydrolyzed by phosphatases within the intestinal lumen (converts to free form)
• carrier-mediated and energy-depended processes absorb the free-form
• after absorption, free riboflavin is transported in the blood and converted back to the coenzyme form (FAD/FMN) and congregates in the liver, kidney, and heart
• coenzyme form are bound to apoenzyme and function as prosthetic groups for oxidation-reduction reactions

Question 18

how is the free form of riboflavin converted back to its coenzyme form after absorption?

Answer 18

ATP-dependent reactions (in tissue cells)

Question 19

how is riboflavin excreted out the body?

Answer 19

primarily in urine with small amount in feces

Question 20

what is the deficiency of riboflavin and what individuals have higher risk?

Answer 20

• ariboflavinosis (acute)
• individuals with homozygous mutation MTHFR, 677TT

Question 21

large doses of riboflavin are known to treat what?

Answer 21

migraine headaches

Question 22

how do we assess nutritional status of B2?

Answer 22

• by measuring the activity of erythrocyte glutathione reductase (if limited activity, an individual has a deficiency)
• AC 1.1.2 = normal
• AC >1.4 = deficiency

Question 23

what are the key enzymes riboflavin is involved in?

Answer 23

• succinate dehydrogenase
• glutathione reductase
• thioredoxin reductase

Question 24

what are the physiological processes riboflavin is involved in?

Answer 24

• electron transport chain
• fatty acid oxidation
• redox reactions