Vitamins (Week 4--Melega)

Question 1

What is a vitamin?

Answer 1

Molecules that humans cannot synthesize

Must be supplied in the diet

Usually are coenzymes for enzymatic reactions

Question 2

Water soluble vs. fat soluble vitamins

Answer 2

Water soluble: taken into tissue until tissues saturated, then the rest is excreted (exception is B12, which we can store a lot of!)

Fat soluble: can accumulate with extra intake

Question 3

What is a mineral?

Answer 3

Term mineral not used much anymore, because really these are ions (Na, Mg, P, S, Cl, K, Ca, Mg, Fe, Co, Ni, Cu, Zn…)

Required for nutririon also

Serve functional/structural roles and serve as electrolytes

Question 4

Enzyme

Answer 4

Catalyst that lowers activation energy of a reaction

Question 5

Coenzyme

Answer 5

Organic nonprotein small molecule that makes up part of the active site of the enzyme and needed for enzyme function (complete the structure of holoenzyme)

Vitamins or vitamin derivatives (NAD+, NADP+, FAD+ which all transfer electrons/H released by catabolic pathways)

Note: if it is NOT organic, then is called a cofactor (metal such as Fe, Mg, Zn)

Question 6

B complex

Answer 6

Water soluble

Energy “fuel” metabolism (glycolysis then TCA)

B1 = thiamine

B2 = riboflavin

Niacin

B6 = pyridoxine

Pantothenate

Biotin

B12 = cobalamin

Question 7

Vitamin C

Answer 7

Water soluble

Antioxidant, regenerates vitamin E

AKA ascorbic acid

Question 8

Folate

Answer 8

Water soluble

DNA synthesis and 1 carbon transfers

AKA folic acid

Question 9

Vitamin A

Answer 9

Fat soluble

Vision

Question 10

Vitamin D

Answer 10

Fat soluble

Ca2+ absorption, bone growth

Question 11

Vitamin E

Answer 11

Fat soluble

Antioxidant

Question 12

Vitamin K

Answer 12

Fat soluble

Coagulation, vascular health

Question 13

B1 (thiamine)

Answer 13

Converted to active form, thiamine pyrophosphate (TPP) in brain and liver

TPP is a coenzyme for:

1) Pyruvate dehydrogenase (PDH): pyruvate –> acetyl CoA for glycolysis and TCA
2) Alpha-ketoglutarate dehydrogenase (alpha-KGDH): alpha-ketoglutarate –> succinyl-CoA for TCA cycle
3) Transketolase: pentose phosphate pathway produces steroids, FAs, AAs, NTs, glutathione

Deficiency disease: peripheral nerve damage (Beriberi); CNS lesions (Wernicke-Korsakoff syndrome); thiamine deficiency common in alcoholics

Question 14

B2 (riboflavin)

Answer 14

Precursor for flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD)

Flavoproteins (enzymes that need FMN or FAD) do redox reactions

Deficiency disease: lesions of corner of mouth, lips, tongue, seborrheic dermatitis

Note: FAD+ is an oxidizing agent (accepts electrons and becomes reduced) and FADH2 is a reducing agent (donates electrons and becomes oxidized)

Question 15

B3 (niacin, or nicotinamide)

Answer 15

Required for synthesis of NAD+ and NADP+, cofactors for many dehydrogenases (lactate dehydrogenase, malate dehydrogenase) for redox reactions

Role in intracellular Ca2+ regulation and cell signaling

Deficiency disease: called pellagra–dermatitis, depressive psychosis, diarrhea (3 D’s for vitamin B3!)

Note: niacin isn’t a true vitamin bc can be synthesized endogenously from tryptophan but is VERY inefficient

Question 16

Consequences of increased NADH/NAD+ ratio (in alcoholics)

Answer 16

Reduced hepatic gluconeogenesis

Decreased fatty acid oxidation

Increased fatty acid synthesis

Reduced activity of TCA (bc slowing rxns that depend on this ratio)

Question 17

B5 (pantothenic acid)

Answer 17

Pantothenate required for synthesis of coenzyme A (CoA) and acyl carrier protein (ACP) domain of fatty acid synthetase

Everything (“pan-“) (70+ enzymes) need CoA to function!

Deficiency disease: peripheral nerve damage

Question 18

B6 (pyridoxine, pyridoxal, pyridoxamine)

Answer 18

Converted to active form pyridoxal phosphate (PLP)

Coenzyme in enzymes for:

1) Transamination: required for synthesis and catabolism of AAs
2) Glycogenolysis: glycogen phosphorylase
3) Biosynthesis of NTs: GABA, 5-HT, DA, NE, EPI
4) Modylation of steroid hormone action

Deficiency disease: disorders of AA metabolism, convulsions

Note: some drugs (isoniazid and penicillamine) complex with pyridoxal and PLP to cause deficiency

Question 19

Biotin (Vitamin H)

Answer 19

Required for enzymes involved in carboxylation (acetyl CoA carboxylase and pyruvate carboxylase)

Carboxylation reactions in gluconeogenesis, fatty acid synthesis, regulation of cell cycle

Deficiency disease: impaired fat and carbohydrate metabolism, dermatitis

Note: consumption of raw eggs can cause biotin deficiency because egg white protein (avidin) binds biotin and prevents intestinal absorption

Question 20

B12 (cobalamin)

Answer 20

Transfer of one-carbon fragments and metabolism of folic acid (does homocysteine –> methioine)

Synthesized only by microorganisms; found in liver and tissues of animals, always bound to protein

Deficiency disease: pernicious anemia (megaloblastic anemia due to inability to do DNA synthesis (block purine and thymidine biosynthesis)), subacute degeneration of spinal cord, polyneuropathy (inhibition of fatty acid synthesis and destruction of myelin sheath)

Note: need intrinsic factor in order to absorb B12 in the ileum

Question 21

How is vitamin B12 (cobalamin) absorbed?

Answer 21

Must be hydrolyzed from protein by gastric acid in stomach –> in ileum, B12 bound by intrinsic factor and absorbed into enterocyte –> vitamin transported to liver in blood bound to transcobalamin II –> delivered to target cells

Note: B12 can be stored in liver for ~6 years

Question 22

Why isn’t B12 degraded by acid in the stomach?

Answer 22

Binds to haptocorrin (R-protein) secreted by saliva

Question 23

What is the significance of B12 catalyzing homocysteine –> methionine reaction?

Answer 23

This reaction is needed for the “methionine synthase” reaction, to create the active form of folic acid (methyltetrahydrofolate –> THF (H4 folate))

Vitamin B12 deficiency causes accumulation of homocysteine and in other reaction, traps folate as methyltetrahydrofolate

Methionine synthase reaction is only known metabolic step where both vitamin B12 and folate are essential

Question 24

What are the two reactions we need vitamin B12 for?

Answer 24

1) Homocysteine –> methoinine (thus methyltetrahydrofolate –> THF), needed for folic acid activation and now can’t do as much DNA synthesis (purine/thymidine biosynthesis blocked)
2) Methylmalonyl CoA –> Succinyl-CoA, causes demyelination/neurological problems because methylmalonyl CoA outcompetes malonyl CoA so malonyl CoA can’t synthesize fatty acids as well

Question 25

Folic acid

Answer 25

Transfer of one-carbon units

Must be converted to tetrahydrofolic acid (THF (H4 folate), but there are many different THF derivatives that work) to be active (mostly reduced in liver)

Need folic acid to do DNA synthesis

Deficiency disease: megaloblastic anemia

Question 26

How exactly does folate deficiency cause disruption of DNA synthesis?

Answer 26

Ultimately need N5, N10-Methylenetetrahydrofolate

To get this, dietary folic acid needs to get into cycle to create this

N5, N10-Methylenetetrahydrofolate needed to make purines directly and to make dTMP (nucleoside thymidine) via thymidylate synthase enzyme (which leads to DNA synthesis) when it continues in the cycle

Question 27

Two ways dietary folates enter THF coenzyme pool/cycle?

Answer 27

1) Vitamin B12 reaction to THF
2) Folate reductase turns folic acid –> dihydrofolate which enters cycle to become THF

Question 28

Other than DNA synthesis, what else are folates required for?

Answer 28

Biosynthesis of methionine and glycine

(Look at THF cycle)

Question 29

What does your body do when it thinks you have a vitamin B12 deficiency?

Answer 29

N5, N10-Methylenetetrahydrofolate (what you want in the end) turns into N5-Methyltetrahydrofolate because that’s the substrate you need to make THF, but the problem is that the missing B12 is what’s needed to make that into THF so this is the “folate trap”

Question 30

How does folic acid and/or vitamin B12 deficiency lead to anemia?

Answer 30

Inability to synthesize DNA during erythrocyte maturation leads to abnormally large erythrocytes (macrocytic/megaloblastic anemia)

Doesn’t cause problems in other cells’ DNA synthesis, weirdly enough…

Question 31

Why do you need folic acid when you’re pregnant?

Answer 31

Increased number of rapidly proliferating cells in the blood

Need at least 600ug/day if intending to become pregnant

Need for folate nearly doubles by third trimester

Question 32

Vitamin C

Answer 32

Hydroxylation of proline and lysine in collagen synthesis; antioxidant; enhances absorption of Fe (Fe3+ –> Fe2+); synthesis of DA, NE, EPI

Active form is ascorbic acid

After it reduces something, dehydroascorbate can be reduced back to active form by enzymes and glutathione

Deficiency disease: scurvy (impaired wound healing, subcut hemorrhage, soft swollen gums, osteoporosis, anemia)

Question 33

Is it good to have lots and lots of vitamins?

Answer 33

If on healthy diet, no need to take extra vitamins

Most coenzymes (vitamins) cause adverse effects if you have too much

Note: vitamin C has hyperbolic curve, transporter function is saturated (active transport and simple diffusion) and after renal resorption threshold, vitamin C escess excreted in urine

Question 34

Why does impaired DNA synthesis cause anemia?

Answer 34

For some reason it really affects RBC maturation and your RBCs get very large (megaloblastic anemia)