riboflavin (b2)
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RIBOFLAVIN (B2]
Gandham. Rajeev
Department of Biochemistry,Akash Institute of Medical Sciences & Research Centre,Devanahalli, Bangalore, Karnataka, India.
E-Mail: gandhamrajeev33@gmail.com
Riboflavin
STRUCTURE
• Riboflavin is a yellow pigment
• Riboflavin contains 6,7 – dimethyl
isoalloxazine ring attached to D-ribitol by
a nitrogen atom
• Ribitol is an open chain form of sugar
ribose with aldehyde group is reduced to
alcohol
• It emits yellow fluorescence
• It is stable to heat but sensitive to light
• When exposed to UV rays of sun light, it is
converted to lumiflavin which exhibits
yellow fluorescence
• Lactoflavin from milk
• Hepatoflavin from liver
• Ovoflavin from eggs are structurally
identical to riboflavin
Structure of Riboflavin
N
NH
O
O
N
N
H3C
H3C
I H - C - OH
IH3C
I H - C - OH
I H - C - OH
ICH2OH
RIBITOL
Riboflavin
ATP
ADP
Flavokinase
Isoalloxine ring
N
NH
O
O
N
N
H3C
H3C
I H - C - OH
IH3C
I H - C - OH
I H - C – OH
ICH2 O - P – O-
FMNATP
PPiFAD synthase
O
O
N
NH
O
O
N
N
H3C
H3C
I H - C - OH
IH3C
I H - C - OH
I H - C – OH
ICH2 O - P – O
FAD
O
O
- P – O
O
O
-CH2
H
N
N
NH2
IN
N
H
• Absorption:
• Riboflavin is present in the food as FAD, FMN
and free riboflavin
• FMN & FAD are hydrolysed to free form in
upper small intestine
• Free form is absorbed by intestinal mucosal
cells by sodium dependent transport system
• Transport:
• In the intestinal mucosal cells riboflavin is
converted into FMN by the action of
flavokinase in the presence of ATP
• FMN enters the portal circulation
• In the plasma it is transported as Albumin-
FMN complex
• FMN complex enters the tissues including liver
• In the tissues it is converted into FAD
• Storage:
• Riboflavin is mainly stored in liver
• It is stored as FMN & FAD
• Excretion:
• Mainly excreted in urine
• Coenzymes of Riboflavin:
• FMN & FAD
• The Ribitol is linked to phosphate in FMN
• FAD is formed from FMN by transfer of an
AMP from ATP
• Biochemical functions:
• FAD & FMN participate in many redox
reactions responsible for energy production
• The functional unit is isoalloxazine ring,
serves as an acceptor of two hydrogen atoms
• FMN or FAD undergo identical reversible
reactions accepting 2H atoms forming
FMNH2 or FADH2
• Flavoproteins:
• The enzymes that use flavin coenzymes are
called as flavoproteins
• Metalloflavoproteins:
• Many flavoproteins contain metal atoms
(iron,molybdenum etc) which are known as
metalloflavoproteins
Reactions requiring FMN
• FMN is the prosthetic group of L – amino acid
oxidase & NADH dehydrogenase
(1) NADH dehydrogenase catalyzes the
transfer electrons from NADH coenzyme
• In this, FMN is involved in transfer of
electrons from NADH to iron sulfur proteins
• Electrons are then transferred to Coenzyme
Q
(2) L – Amino acid oxidase:• It catalyzes the conversion of L-amino acid to
the α-ketoacid • Ammonia is released & FMN is reduced to
FMNH2
NADH + H+ + FMN
NAD + FMNH2
NADH Dehydrogenase
L-Amino acid + FMN
α- Keto acid + NH3 + FMNH2
L – amino acid oxidase
Reactions requiring FAD
• Enzymes containing FAD
• Carbohydrate metabolism:
• PDH Complex:
• PDH complex catalyzes the oxidative
decarboxylation of pyruvate to acetyl CoA
• NAD is reduced to NADH + H+
• FAD is present in Dihydrolipoyl
dehydrogenase of PDHPyruvate
Acetyl CoA
PDH ComplexFAD
NAD
NADH + H+
• α- Ketoglutarate dehydrogenase complex:
• It catalyzes the oxidative decarboxylation of
α- Ketoglutarate to succinyl CoA
• FAD is present in dihydrolipoyl
dehydrogenase of α- Ketoglutarate
dehydrogenase complex
α- Ketoglutarate
Succinyl CoA
α- KetoglutarateComplexFAD
NAD
NADH + H+
CoA SH
CO2
• Succinate dehydrogenase:
• It catalyzes the oxidation of succinate to
fumarate
• FAD is reduced to FADH2
Succinate
FAD
Fumarate
Succinate Dehydrogenase
FADH2
LIPID METABOLISM
• Acyl CoA Dehydrogenase:
• It catalyses the Oxidation of fatty acyl CoA
to 2, 3 unsaturated acyl CoA
Fatty acyl CoA
2, 3 unsaturated acyl CoA
Acyl CoA dehydrogenaseFAD
FADH2
Mitochondrial Glycerol 3-P dehydrogenase
• It catalyzes the conversion of glycerol 3P to
DHAP in mitochondria
• It essential for carrying reducing equivalents
from cytosol to mitochondria
Glycerol 3-phosphate
DHAP
Mitochondrial glycerol 3-P dehydrogenase
FAD
FADH2
Protein metabolism
• Glycine cleavage system:
• It catalyzes the conversion of glycine to CO2
and ammonia
• FH4 is converted into N 5,10 methylene FH4
Glycine + FH4
N 5,10 methylene FH4 + CO2 + NH3
Glycine cleavage system
D – Amino acid Oxidase
• It catalyzes the conversion of D – amino
acids or glycine to corresponding ketoacids
• Ammonia is reduced
• FAD is reduced to FADH2
D – Amino acid + FAD
α - Keto acid + NH3 + FADH2
D – Amino acid Oxidase
Purine metabolism
• Xanthine Oxidase:
• It catalyzes the oxidation of hypoxanthine
to xanthine and xanthine to uric acid
• Xanthine oxidase contains FAD,
molybdenum and ironHypoxanthine
Xanthine
Xanthine oxidaseFAD
FADH2
Uric acid
Xanthine oxidase
Dietary sources
• Rich sources are milk, milk products,
meat,eggs, liver and kidney
• Moderate sources are cereals, fruits,
vegetables and fish
• RDA:
• Men - 1.5 mg/day
• Women - 1.2 mg/day
• Pregnancy & lactation - 1.5 mg/day
+ Deficiency
• Causes:
• Inadequate intake
• Impaired absorption due to intestinal diseases
• Chronic alcoholics are susceptible to B2
deficiency
• Clinical features:
• Cheilosis (fissures at the corners of mouth)
• Glossitis (tongue smooth and purplish)
• Dermatitis
• Corneal vasculalarization: includes dryness,
burning & itching and lacrimination
• Measurement of glutathione reductase in
erythrocytes is a reliable diagnostic test to
assess riboflavin deficiency
• Reference interval
• Serum or plasma level is 4 to 24 µg/ dl
Dermatitis, Riboflavin deficiency
References
• Harper’s Biochemistry 25th Edition.
• Fundamentals of Clinical Chemistry by Tietz.
• Text Book of Medical Biochemistry-A R Aroor.
• Text Book of Biochemistry-DM Vasudevan
• Text Book of Biochemistry-MN Chatterjea
• Text Book of Biochemistry-Dr.U.Satyanarana
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