Medical Biochemistry 2: Iron, Folate & Vitamin B12 Metabolism

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Questions and Answers

What percentage of iron in the body is stored in blood?

75% (correct)

60%

90%

50%

What is the main function of iron in the human body?

Energy production

Immune system function

Protein synthesis

Oxygen transportation and basic metabolic reactions (correct)

What is the daily iron requirement for pregnant women?

30 mg/day

20 mg/day

10 mg/day

40 mg/day (correct)

Where does iron absorption occur mainly?

Duodenum and upper jejunum Signup and view all the answers

What is the only form of iron that is absorbed?

Fe2+ (ferrous) Signup and view all the answers

Why are patients with thalassaemia and sickle cell anemia at risk for iron overload?

Due to increased intestinal iron absorption and body iron overload Signup and view all the answers

What is heme found mostly in?

Hemoglobin, myoglobin and cytochromes Signup and view all the answers

What is the function of hepcidin in the body?

To regulate iron transport and prevent excess iron absorption Signup and view all the answers

What is the treatment for Hemosiderosis?

Phlebotomy and Desferoxamine Signup and view all the answers

What is the recommended daily allowance of folic acid?

400 µg/day Signup and view all the answers

How is folic acid transported in the blood?

By beta globulins Signup and view all the answers

What happens to folic acid when it is exposed to light?

It is rapidly destroyed Signup and view all the answers

What is the primary function of transferrin in the circulation?

Transport iron in the circulation to sites where iron is required Signup and view all the answers

What is the mechanism by which iron is released from enterocytes into the bloodstream?

Ferroportin Signup and view all the answers

What is the daily loss of iron in the body due to?

Exfoliated epithelial cells of the skin and GI tract Signup and view all the answers

How is iron homeostasis maintained in the body?

By regulating iron absorption Signup and view all the answers

What signals the mucosa to increase iron absorption in response to anemia?

Erythropoietin from the kidney Signup and view all the answers

What is the outcome when the concentration of iron is high?

Ferritin synthesis is increased Signup and view all the answers

What is the primary source of vitamin B12?

Foods of animal origin Signup and view all the answers

What is the recommended daily allowance of vitamin B12?

1-2 µg/day Signup and view all the answers

What is the functional co-enzyme form of vitamin B12 in the body?

Methylcobalamin Signup and view all the answers

What is the role of intrinsic factor in vitamin B12 absorption?

Binding to vitamin B12 in the duodenum Signup and view all the answers

What is the protein that binds to vitamin B12 in the circulation?

Transcobalamin (TC-l & TC-ll) Signup and view all the answers

What is the fate of excess methylcobalamin in the body?

It is stored in the liver as deoxyadenosylcobalamin Signup and view all the answers

In which form does folic acid exist when stored in the liver or ingested?

Polyglutamate form Signup and view all the answers

What is the function of the folate conjugase in the intestinal mucosal cells?

To remove glutamate residues from folic acid Signup and view all the answers

What is the carrier of one-carbon groups in the body?

Tetrahydrofolic acid (THFA) Signup and view all the answers

What is the most common circulating form of the vitamin in the body?

5-methyltetrahydrofolic acid Signup and view all the answers

What is the major function of the folic acid co-enzyme?

The transfer of one-carbon group Signup and view all the answers

What is the methyl group in N5-methyl THFA used for?

Synthesis of active methionine Signup and view all the answers

What is one of the causes of folic acid deficiency?

Increased requirement Signup and view all the answers

What is the function of S-adenosyl methionine (SAM) in transmethylation reactions?

Methyl group donor Signup and view all the answers

Study Notes

Iron Metabolism

Iron is an essential nutrient for the human body, with approximately 3-5g stored in the body (75% in blood, 25% in bone marrow, liver, and muscles).
Iron plays a crucial role in oxygen transportation and basic metabolic oxidative and reductive reactions.
Dietary sources of iron include organ meats, fish, oysters, egg yolks, dried beans, dried figs and dates, and green vegetables.
Heme iron is found in animal products, while non-heme iron is found in vegetables.
Daily iron requirements are: 5-10 mg/day for males, 20 mg/day for females, and 40 mg/day for pregnant women.

Iron Absorption

Iron absorption occurs mainly in the duodenum and upper jejunum.
Only the ferrous (Fe2+) form of iron is absorbed.
Divalent metal transporter (DMT-1) facilitates the transfer of iron across the intestinal epithelial cell.
Iron is released into the bloodstream via ferroportin and binds to transferrin.

Iron Excretion

There is no physiological regulated mechanism for iron excretion.
Approximately 1-2 mg of iron is lost daily through exfoliated epithelial cells of the GI tract, skin, and menstruation.

Regulation of Iron Absorption

Iron metabolism is regulated at the level of absorption, not excretion.
Mucosal regulation: when iron stores are depleted, absorption is enhanced, and when iron stores are adequate, absorption is decreased.
Stores regulation: when body iron stores fall, the mucosa is signaled to increase absorption.
Erythropoietic regulation: in response to anemia, the RBCs signal the mucosa to increase iron absorption.
Control of ferritin and transferrin receptor synthesis: when iron concentrations are high, ferritin is synthesized to store iron.

Disorders of Iron Metabolism

Iron deficiency: inadequate iron stores, leading to anemia and impaired oxygen delivery.
Iron overload: excessive iron absorption, leading to tissue damage and organ dysfunction.
Hereditary hemochromatosis: a genetic disorder characterized by increased iron absorption and transferrin levels in the serum, leading to iron overload.
Hepcidin: a hormone that regulates iron transport, preventing excess iron absorption and maintaining normal iron levels.

Folic Acid/Folate (Vitamin B9)

Folic acid is composed of three components: pteridine ring, para-aminobenzoic acid (PABA), and glutamic acid residues.
Folic acid is soluble in water but is rapidly destroyed when exposed to light.
Recommended daily allowance (RDA) of folic acid: 400 µg/day for adults, 600 µg/day for pregnant women, and 500 µg/day for lactation.

Absorption of Folic Acid

Folic acid is readily absorbed by the upper part of the jejunum.
In the blood, it is transported by beta globulins and taken up by the liver, where co-enzymes are produced.
Intestinal mucosal cells remove some of the glutamate residues through the action of folate conjugase, making folate less negatively charged and more capable of passing through the basal luminal membrane.

Co-enzyme of Folic Acid

The most common circulating form of the vitamin is 5-methyltetrahydrofolic acid.
The major function of folic acid co-enzyme is the transfer of one-carbon groups in a variety of synthetic reactions.
The methyl group in N5-methyl THFA is used for the synthesis of active methionine, which takes part in transmethylation reactions.

Vitamin B12

Foods of animal origin are the only sources of vitamin B12, with rich sources including liver, kidney, milk, curd, eggs, fish, pork, and chicken.
Recommended daily allowance (RDA) of vitamin B12: 1-2 µg/day for adults, 2 mg/day for pregnant women and lactation.

Absorption of Vitamin B12

Absorption of vitamin B12 requires two binding proteins: intrinsic factor (IF) and cobalophilin/Haptocorrin.
Intrinsic factor is secreted by gastric parietal cells, while cobalophilin/Haptocorrin is secreted by the salivary gland.
Gastric pepsin releases the vitamin from proteins of food, and then it binds with cobalophilin. In the duodenum, cobalophilin is hydrolyzed by trypsin, and the vitamin binds to intrinsic factor. The whole IF-B12 complex is endocytosed and transported to the liver, where it is converted to methylcobalamin and stored.

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Description

This quiz covers the metabolism of iron, folate, and vitamin B12, including absorption, storage, transportation, excretion, and disorders related to iron metabolism. It is part of the Medical Biochemistry 2 course.