Thiamin

Question 1

Thiamin History

Answer 1

• First described as water-soluble vitamin
• 1884: Dr. Takaki hypothesized that beriberi was caused by dietary insufficiency
• 1897: Christiaan Eijkman – Paralysis in birds fed cooked, polished rice. Reversed when rice polishing stopped (Nobel prize)
• 1901: essential nutrient in the outer layer of the grain
• ’20s and ‘30s: isolation, structure and synthesis of thiamin: “sulfur-containing vitamin”

Question 2

Properties of thiamin

Answer 2

• colorless
• water soluble
• not stable in UV light, moisture, alkaline/neutral solutions

Question 3

Structure and active site of thiamin

Answer 3

the double bond of the thiazole

Question 4

What is the active form of thiamin act as?

Answer 4

A co-enzyme

Question 5

What is the active form of thiamin?

Answer 5

Thiamin pyrophosphate (or diphosphate) so it has 2 phosphates attached to the alcohol

Question 6

TPP/TDP

Answer 6

thiamine pyrophosphate/diphosphate

Question 7

Properties of the active form of Thiamin

Answer 7

• TPP
• can also exist as mono- and tri-phosphate forms (but these are not active)
• thiaminylated adenines (nucleotides)

Question 8

Metabolic role of thiamin

Answer 8

Coenzyme in >24 enzyme that plays a role in:
* Nervous system function (role in nerve conduction & in neural membranes)
* Energy production
* Biosynthesis of lipids

Question 9

What vitamins act as coenzymes?

Answer 9

• B vitamins
• vitamin C
• vitamin K

Question 10

How does coenzyme action work?

Answer 10

Bind to enzymes which enable them to perform activity to either synthesize compounds or dismantle them. Without coenzymes the substrate do not respond, so the vitamin creates an attractive force so that the enzyme recognizes the substrates

Question 11

Metabolic reactions TPP is involved in

Answer 11

• oxidative decarboxylations of α-keto acids
• transketolation
• α-oxidation of phytanic acid

Question 12

Enzymes for oxidative decarboxylations of α-keto acids of which TPP is a co enzyme

Answer 12

Involved in breaking down metabolites for energy
* pyruvate dehydrogenase complex
* α-ketoglutarate dehydrogenase
* branched-chain α-keto acid dehydrogenase

Question 13

What does PDH do?

Answer 13

Makes Acetyl CoA from pyruvate which then enters into the CAC; NAD+ →NADH, releases CO2
* E1: pyruvate dehydrogenase
* E2: Dihydrolipoyl transacetylase
* E3: Dihydrolipoyl dehydrogenase

Question 14

Role of TPP in PDH

Answer 14

Involved in the first E1 with pyruvate dehydrogenase where TPP accepts and donates acetyl groups from pyruvate and CO2 is released.
* The acteyl binds to the active site of TPP

Question 15

What does α-ketoglutarate dehydrogenase do?

Answer 15

Part of the CAC, converting α-ketoglutarate → Succinyl CoA; NAD→NADH, releases CO2

Question 16

Role of TPP in α-ketoglutarate dehydrogenase

Answer 16

Present on the enzyme and the active site is what accepts the carbon structure so that it can be transferred to CoA-S

Question 17

What does Branched-chain α-keto acid dehydrogenase do?

Answer 17

Enzymes that breaks down AAs so we can get energy from them
* Deamination
* TPP dependant decarboxylation

Question 18

Role of TPP in Branched-chain α-keto acid dehydrogenase.

Answer 18

Required in the α-keto acid decarboxylation and without it there would be a build up of branched chain AAs and could not produce energy

Question 19

What is transketolation?

Answer 19

Interconversion of sugar phosphates in in the pentose phosphate pathway
* The enzyme transketolase is a way we get the sugars we need in our cells

Question 20

Role of TPP in transketolation

Answer 20

TPP moves the carbons around to make new sugars
* TPP is attached to transketolase and a 2 carbon structure from the sugar attaches to the TPP and depending on enzyme present the 2-C structure will transfer to a sugar and make a new one by adding on two Carbons

Question 21

From the PPP what is ribose-5-phosphate and the NADPH go towards?

Answer 21

• ribose-5-phosphate: nucleic acids, complex sugars, coenzymes
• NADPH: coenzymes, steroids, fatty acids, amino aicds, neurotransmitters and glutathione

Question 22

Role of thiamin in 𝝰-oxidation of phytanic acid?

Answer 22

Thiamin is attached to the enzyme phytanoyl-CoA hydrolase which gets rid of the methyl group of phytanic acid to prevent build up since it cannot undergo β-oxidation

Question 23

Location of PDH, KDH, BCKDH, and transketolase

Answer 23

• PDH, KHD, BCKDH: mitochondria
• Transketolase: cytosol

Question 24

What is phytanic acid?

Answer 24

3-methyl-substituted fatty acid which we don’t usually make but it is found in meat, dairy and fish which cannot undergo β-oxidation

Question 25

What happens with a disorder of the phytanoyl-CoA hydrolase enzyme?

Answer 25

Can cause buildup of phytanic acid and lead to Refsum’s disease * A rare genetic disease that causes weakness or numbness of the hands and feet (peripheral neuropathy)

Question 26

How is thiamin absorbed?

Answer 26

In our diet it is often in the active form because that is how it is present in the foods. We dont absorb large molecules or those will lots of charge very well, however. It is first broken down to its vitamin form, so the phosphates of TPP are removed via pyrophosphatases. Thiamin has a transporter into the enterocyte which is the main route and then some diffusion. Once in the enterocyte can be activated to TPP (via TPK) and used within the enterocyte, but most will be absorbed into circulation and hit liver first to be used. What the liver does not need leaves and goes to other tissues especially muscle which uses the most thiamin since it uses a lot of energy.

Question 27

How is thiamin present in circulation vs. cells?

Answer 27

* plasma: free thiamin and TMP * cellular compartment: TPP

Question 28

How does the thiamin remain in a cell?

Answer 28

metabolic trapping * Adding the phosphate groups gives the charge so they cant leave

Question 29

Thiamin storage

Answer 29

only small amount (~30 mg) stored in the body which is mostly in skeletal muscle (50% body thiamin), as well as liver, kidney, & nervous tissues * 80% as TPP, 10% TTP, rest as thiamin & TMP * high metabolic rate + low storage amounts = daily intake necessary

Question 30

Why of the thiamin found in the muscle?

Answer 30

Most of our energy is burned here

Question 31

Thiamin excretion

Answer 31

* Thiamin consumed in excess of tissue needs is rapidly excreted; water soluble so easy to excrete * Reabsorbed in renal brush border membrane via saturable carrier-mediated transport system; transporters are good at maintaining levels where we need it.

Question 32

Where else might thiamin come from besides diet?

Answer 32

Several metabolites arise from the action of gut microflora * Not a significant producer | Need to confirm what this is rtalking about

Question 33

What are the sources of thiamin?

Answer 33

* plants: synthesizedbyplantsandsomemicro- organisms (not animals) * animals: Not synthesized but stored in the muscle so the phosphorylated form

Question 34

plant sources of thiamin

Answer 34

* unrefined/whole grains, nuts, legumes * Enriched flour, grains, bread, cornmeal

Question 35

animal sources of thiamin

Answer 35

Will need to be dephosphorylated * organ meat and (lean) pork * milk and eggs

Question 36

What cooking process reduces thiamin?

Answer 36

Boiling/blanchingreducesthiamin

Question 37

What processes can destroy thiamin?

Answer 37

* baking soda * UV * Thiaminases (in raw freshwater fish, shellfish and ferns) * Heat-stable antagonist (caffeic acid and tannic acid; found in coffee, tea, Brussels sprouts, rice bran) * Sulfite (high in processed food - preservative)

Question 38

What determines thiamin requirements?

Answer 38

Related to total energy intake * Involved in metabolism of CHO, lipids and proteins

Question 39

When might elevated thiamin be seen?

Answer 39

those with high caloric requirements: * e.g. patients on dialysis, long-term IV-feeding and chronic infection

Question 40

How can thiamin requirements be altered?

Answer 40

* Thiaminases - lower * Thiamin antagonists - lower * Vitamin C: may counteract anti-thiamin activity of tannic acid * Higher metabolic rate may require more

Question 41

RDAs for thiamin

Answer 41

0.4 mg/1000 kcal/day (for 2000+ kcal diets) but no less than 1.0 mg * no UL set due to lack of reported adverse effects

Question 42

How to determine thiamin status

Answer 42

* urinary excretion * blood pyruvic acid or α-ketoglutarate * transketolase activity

Question 43

What is urinary excretion to determine thiamin status typically used for?

Answer 43

Mostly used for rough estimations of a population but not usually the individual

Question 44

How does blood pyruvic acid or α-ketoglutarate determine thiamin status?

Answer 44

↑ in thiamin deficiency * problem = not sensitive enough to ID early deficiency & is non-specific