lec 1 (mcbride) - vitamins and coenzymes

water soluble vs fat soluble vitamins

water soluble

1. vitamin C

2. B vitamins

   1. thiamin (B1)

   2. riboflavin (B2)

   3. niacin (B3)

   4. biotin

   5. pantothenic acid

   6. vitamin B6

   7. folate

   8. vitamin B12

fat soluble (ADEK)

1. vitamin A

2. vitamin D

3. vitamin E

4. vitamin K

water soluble vitamins

• hydrophilic molecules

• limited tissue storage

• rapidly cleared in urine

fat soluble vitamins

• hydrophobic molecules

• stored in fatty tissue and liver

• slow clearance

enzymes function

accelerate the rate of rxns

enzymes require…

require cofactors for their catalytic activity

• enzymes function = limited by the chemical properties of the encoded amino acids (enzymes = proteins)

• enzymes often use nonprotein molecules to perform rxns impossible to do with AA sidechains alone

  • nonprotein molecules = cofactors

what makes up a holoenzyme

holoenzyme = complete, active form of enzyme

apoenzyme (protein portion of enzyme, inactive on its own) + cofactor (non protein factor required for enzymatic activity)

cofactors are divided into 2 groups…

• metal ions (Mg2+, Fe2+, Zn2+)

  • bound to the active site of enzymes to facilitate formation of the transition state by tuning the positioning of the rxn substrates, typically with their pos. charge

• coenzymes

  • small organic molecules which can be:

    • tightly bound and chemically unchanged by the catalyzed reaction (prosthetic groups)

    • loosely associated and behave as second substrate so must be present in stoichiometric ratios with other substrates

B vitamins produce…

many coenzymes critical for cellular metabolism

pyruvate and TCA cycle

• pyruvate dehydrogenase complex links glycolysis to TCA cycle

• for pyruvate to enter TCA it needs to be converted into acetyl coA

  • requires multiple coenzymes

    • thiamine pyrophosphate (TPP) (prosthetic)

    • NAD+ (stoichiometric)

    • CoA (stoichiometric)

what is collagen

• most abundant protein in mammals

• accounts for 25-30% of total protein mass

• main fibrous component of skin, bone, tendon, cartilage and teeth

collagen is extremely rich in…

• glycine, proline, and hydroxyproline

• AA of part of collagen chain reveals that every third residue = glycine

collagen formation

• 3 helical polypeptide chain - each in an extended conformation

• hydrogen bonds within strand of collagen helix are absent

• helix is stabilized by steric repulsion of the pyrrolidine rings of the proline and hydroxyproline residues

• forms a rope-like superhelix

  • consists of 3 intertwined helical polypeptide strands

  • stabilized by hydrogen bonds between strands

  • interior is crowded; only glycine can fit

what is required for collagen production

vitamin C

hydroxyproline

is a modified AA formed as part of collagen synthesis

• procollagen molecules = synthesized containing unmodified proline residues

• many proline residues of procollagen polypeptide → hydroxylated

• hydroxyl group of hydroxyproline form interchain hydrogen bonds that help stabilize the triple-stranded helix

hydroxylation of proline requires…

ascorbate

• ionized form of vitamin C → originally ascorbic acid and then one of the OH becomes O- (ionized) (ascorbate)

ascorbate salt

“improved” vitamin C supplements = often an ascorbate salt

what does each hydroxylation of proline do?

each hydroxylation of proline residue converts ascorbate → dehydroascorbic acid

ascorbic acid deficiency

• deficiency of ascorbic acid aka vita C causes scurvy

  • common in sailors until 19th century

• prevents hydroxylation of proline residues of collagen

  • ascorbate regenerates active prolyl-hydroxylase enzyme

  • NO ascorbate → build up of inactive enzyme

  • lack of active enzyme decreases hydroxylation of collagen proline residues

loss of proline hydroxylation …

decreases stability of newly synthesized collagen

• the loss of OH groups → fewer interchain hydrogen bonds → collagen triple-stranded helix is less stable and weaker

deficiency of vit c & collagen results

as collagen is replaced, tissue loses structure and rigidity → scurvy

• in healthy tissues, collagen is continually degraded and replaced

• complete collagen turnover time can be months or years → depends on tissue

• as more defective collagen (lacking hydroxyproline) accumulates in tissues → scurvy symptoms develop and worsen

in a pt with scurvy, which of the following treatments is more effective than supplementation of vitamin C alone?

D. none of the above

explanation: the key part of the sentence is more effective than supplementation of vitamin C alone

supplementing with hydroxyproline AA CANNOT be incorporated into the synthesis of the polypeptide collagen, you have to incorporate the unmodified proline into polypeptide of collagen and then proline hydroxylase will hydroxylase it once its part of the polypeptide

thiamine (vit B1) deficiency

causes beriberi

• neurologic and CV disorder

• symptoms of beriberi

  • swelling in abdomen and legs

  • pain in limbs

  • muscle weakness

  • mental confusion

  • heart enlargement & other cardiac issues

thiamine is the precursor…

for synthesis of the coenzyme thiamine pyrophosphate (TPP)

TPP is critical for…

central carbon metabolism

• glycolysis

• TCA cycle

• AA catabolism

TPP is the coenzyme for…

pyruvate dehydrogenase and α-ketoglutarate dehydrogenase

• TPP = coenzyme for enzyme pyruvate dehydrogenase

  • pyruvate → acetyl CoA

• TPP = coenzyme for enzyme α-ketoglutarate dehydrogenase

  • α-ketoglutarate → succinyl CoA

in a blood sample from a pt with beriberi, what changes would you predict compared to a healthy person?

B: elevated pyruvate and α-ketoglutarate

explanation: pts with beriberi have a thiamine deficiency which is the precursor for TPP so lack of TPP means that pyruvate and α-ketoglutarate would NOT be able to proceed forward with their rxn into becoming CoA

symptoms of mercury and arsenic poisoning are similar to beriberi due to…

inhibition of pyruvate dehydrogenase complex

treatments for mercury and arsenic poisoning

sulfhydryl compounds that outcompete binding for the metal ion to restore the enzyme

cobalamin (vit b12) deficiency

causes anemia

• blood cells require cobalamin for proliferation

• low cobalamin levels cause:

  • enlarged RBCs

  • nuclear hypersegmentation of DNA in neutrophils

cobalamin structure

• has a complex structure and forms a few similar coenzymes

  • coenzyme B12

  • cyanocobalamin

  • methylcobalamin

cobalamin catalyzes…

a rearrangement reaction

• group on one carbon is exchanged with a proton on an adjacent carbon

• R group can be amino group, OH group or substituted carbon

cobalamin is coenzyme for 2 rxns in mammals

• methylmalonyl-CoA mutase as part of fatty acid degradation

• methionine synthase for remethylation of homocysteine

• cobalamin deficiency disrupts folate cycle → important for nucleotide synthesis and cell proliferation

methylation of homocysteine by methionine synthase

• homocysteine → methionine (methylated version)

• methyl-B12 → B12

in a blood sample from a pt with a cobalamin deficiency, what changes would you predict compared to a healthy person?

D: elevated homocysteine (and methylmalonic acid)

explanation:

cobalamin is a coenzyme for methionine and methylmalonyl-CoA mutase which convert homocysteine and methylmalonic acid → their products

w/out cobalamin, this wouldn’t occur so high levels of the precursors

vitamins must be obtained from __ in humans

diet

• both E. coli and humans use vitamins to produce coenzymes for metabolic rxns yet e. coli can thrive on glucose and organic salts alone while humans need at least 13 vitamins in their diet why?

  • e. coli have biosynthetic pathways to synthesize their vitamins while humans (and mammals) do NOT

    • biosynthetic pathways for vitamins are complex and it is more efficient to ingest vitamins than to synthesize the enzymes required to make vitamins from simple molecules

    • efficiency comes at cost: dependence on other organisms for chemicals essential for life

are there 12 or 13 vitamins required for humans?

13 vitamins listed on slide

nuance due to vitamin D b/c we can synthesize it so its not considered a vitamin sometimes

vitamin D can be synthesized from…

cholesterol by UV light from sunlight exposure

• Vitamin D3 is formed from cholesterol in a series of steps, one of which requires ring-splitting by UV light

• vitamin D3 is converted to the hormone calcitrol, active form of vitamin D, by hydroxylation rxns in liver and kidneys

• vitamin D3 = critical for absorbance of calcium and phosphorus → required for bone health

synthesis of vit D is NOT sufficient for demand so dietary vit D is essential

subset of amino acids

similar to vitamins, a subset of AA must also be obtained from diet

• essential AA must be supplied in the diet

  • histidine

  • isoleucine

  • leucine

  • lysine

  • methionine

  • phenylalanine

  • threonine

  • tryptophan

  • valine

• nonessential AA can be synthesized from other nutrients

  • carbon skeletons of each AA come from intermediates of glycolysis, pentose phosphate pathway, or TCA cycle

  • alanine

  • arginine

  • asparagine

  • aspartate

  • cysteine

  • glutamate

  • glutamine

  • glycine

  • proline

  • serine

  • tyrosine

microorganisms can synthesize…

the entire set of 20 AA yet humans have lost some of these biosynthetic routes

• AA can be sorted into biosynthetic families

• major metabolic precursors = blue

• AA that give rise to other AA are in yellow

• essential AA are in bold

essential AA require...

a greater number of biosynthetic steps that have been lost in humans

• essential and nonessential AA can be distinguished by required # of biosynthetic steps

• some AA are nonessential b/c they can be biosynthesized in small # of steps

• AA requiring a large # of steps for their synthesis are essential in the diet b/c some of the enzymes for these steps have been lost in the course of evolution