lecture 11: riboflavin and thiamin

Question 1

riboflavin (vitamin B2)

Answer 1

required for a variety of metabolic functions including FA oxidation, ETC, DNA replication/repair, etc

Question 2

2 physiological flavo-coenzymes derived from riboflavin

Answer 2

FMN (phosphorylated form) and FAD (synthesized from FMN by addition of AMP)

most flavin coenzymes are non-covalently associated

Question 3

isoalloxazine ring of FAD and FMN

Answer 3

considered the most chemically versatile redox cofactor in the cell
1 OR 2 electron transfers, capable of semiquinone state

Question 4

cytochrome P450 and FAD

Answer 4

functions in the metabolism of various substrates

accepts electrons, reduces Fe3+ to Fe2+

NADPH becomes oxidized, reduced FAD to FADH2 > 2 ferredoxins

Question 5

semiquinone state

Answer 5

capable of accepting and donating single electron - intermediate oxidative state

due to isoalloxazine ring in FMN/FAD

Question 6

riboflavin deficiency

Answer 6

resembles niacin deficiency

stunted growth, skin lesions

Question 7

food sources of riboflavin

Answer 7

we cannot produce but we can store and consume - no de novo synthesis pathway
mainly in coenzymatic forms, typically as FAD

Question 8

thiamin diphosphate / ThDP/ TPP

Answer 8

two phosphates are attached to hydroxyl group of thiamin > active form of cofactor

can be converted to back to ThTP or ThMP

Question 8

thiamin deficiency

Answer 8

beriberi
fast heart rate, SOB, numbness, etc

Question 9

thiamin function

Answer 9

synthesis of nucleotides
1) decarboxylations
2) transfer of 2C fragments from ketose to aldose

Question 10

thiamin in the decarboxylation of the pyruvate dehydrogenase complex

Answer 10

TPP forms a carbanion with pKa 10 (relatively acidic)
TPP carbanion does nucleophilic attack of pyruvate carbonyl group
decarboxylation after TPP and pyruvate binding

TPP carbanion also responsible for ketose to aldose

Question 10

Riboflavin’s isoalloxazine ring structure is vital to its function in redox reactions. What are riboflavin-derived cofactors capable of doing that niacin-derived cofactors cannot?

Answer 10

Riboflavin-derived cofactors, such as FMN and FAD, are capable of one or two electron transfers and can exist in 3 redox states. Therefore, these cofactors are capable of being in an intermediate semiquinone state and performing just one electron transfer.

Question 11

It is not uncommon for the deficiency of one B vitamin to resemble the deficiency of another B vitamin. For example, the deficiency of niacin, riboflavin, and thiamin have many similarities. Why is this so? In addition, name a biological process that is dependent upon all three of the named B vitamins.

Answer 11

All three vitamins function the same pathways. if one is not present the entire pathway does not work. For example, they are all required for the pyruvate-dehydrogenase complex and the Citric Acid Cycle