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Transcript
Section 10: Nutrients and their functions • B vitamins 02/03/06 B Vitamins • The water soluble B-complex vitamins and several related nonvitamins are carriers in group transfer reactions. Entity Transferred electron (H’) electron (H’) electron (H’) electron aldehyde acyl group acyl group, electron alkyl group carbon dioxide amino group methyl, methylene, or formyl group 1 Coenzyme NAD, NADP FMN, FAD coenzyme Q heme derivatives thiamine pyrophosphate coenzyme A lipoyl (-enzyme) coenzyme B12 biotin (-enzyme) pyridoxal phophate tetrahydrofolate Vitamin niacin (nicotinamide) riboflavin (B2) thiamine (B1) pantothenic acid B12 (cobalamine) biotin pyridoxine (B6) folic acid (folate) Overview of B Vitamin Deficiency • Early studies of beriberi and pellagra focused on something present in starch that caused the diseases. Later it was learned that the symptoms were due to something missing. • The B-complex vitamins are missing in refined foods (white bread, white rice), which have had the metabolically active portions of the whole grain removed. • These vitamins are cofactors for a large number of reaction schemes that derive energy from food and produce essential biosynthetic intermediates and products. • The symptoms of deficiencies are not unique, and it is difficult to assign a given symptom to a particular missing vitamin. • Currently, refined foods typically have the B-complex vitamins added back, and deficiencies are seen only among those with the most impoverished diets. 2 O Niacin O O NH2 O O O P O CH2 N+ H HO OH N+ H Niacin • RDA = 15 & 19 mg (F&M). O NH2 • The vitamin niacin is N converted to nicotinamide and N incorporated into electron N N carriers for catabolic and O O P O CH2 anabolic metabolism. H O • Deficiency, when its precursor OR HO tryptophan is also low, leads NAD+ (R=H) & NADP+ (R=PO3=) to pellagra (dermatitis, stomatitis, glossitis). 3 O Riboflavin O H3C N H3C N H3C N NH O HC OH O HC OH HC OH HC OH HC OH H2C HC OH N H2C N O O O P O N O P O CH2 O FAD 4 N H2C NH H2C O H3C H HO OR OH Riboflavin N N • RDA = 1.3 & 1.7 mg (F&M). • Incorporated into electron carriers (free and proteinbound). • Deficiency leads to cheilosis. Coenzyme Q O H3C O CH3 H3C O ( CH3 )10 O Coenzyme Q (oxidized form) • Not a vitamin. • An electron carrier in the mitochondrial inner membrane. 5 H CH3 Heme Derivatives S H3C CH3 H3C O O • Hemes are not vitamins. • They have roles as electron carriers and O oxygen carriers. O 6 N N Fe N N S CH3 Thiamine NH2 + N N • RDA = 1.1 & 1.5 mg (F&M). H3C N • The vitamin thiamine is pyrophosphorylated to make a coenzyme for several aldehyde transfer reactions in carbohydrate metabolism. • Deficiency causes beriberi (light-sensitive weak eyes, purple tongue). + N N N H3C S O CH2 CH2 O P O Thiamine pyrophosphate 7 O O CH2 CH2 OH Thiamine reactive carbon NH2 H3C H3C S P O O Pantothenic Acid O OH CH3 O C CH2 CH2 N C CH O H C CH2 OH CH3 Pantothenic Acid • Safe and adequate daily dietary intake = 4 - 7 mg. • The vitamin is incorporated into coenzyme A and acyl carrier protein of the fatty acid synthase complex. • No deficiencies known. NH2 HS N H2C CH2 O OH CH3 N C CH2 CH2 N C CH C CH2 O O CH3 H Coenzyme A 8 O O N O P O P O CH2 H O O OH HO N N O H Lipoic Acid O S S Lipoate • Not a vitamin. • Lipoic acid is covalently attached to active sites by a peptide linkage to a lysine. • An acyl and e- carrier. H O H N H N S S reactive disulfide 9 O Lipoamide • RDA = 2 mg (F&M). • Absent in plants. • Required for methyl transfers, DNA synthesis, succinyl CoA synthesis. • Deficiency, almost always due to inadequate production of a glycoprotein (intrinsic factor) in the stomach lining, causes pernicious anemia. 10 Cobalamin 5'-deoxyadenosine H2N O O H2N H3C NH2 O O NH2 H3C N H3C N N Co N O CH3 CH3 N CH3 CH3 dimethylbenzimidazole Coenzyme B 12 NH2 O O Biotin HN • Safe and adequate daily dietary intake = 30 - 100 mg (F&M). • Required for CO2 transfers, malonyl CoA synthesis, oxaloacetate synthesis. • Deficiency rare, although it can be induced by high levels of avidin in the diet, because avidin binds biotin very tightly. O NH Biotin O S O reactive nitrogen HN NH lysine side chain HN N S O 11 enzyme O OH H2C HO Pyridoxine H3C O HC HO O N • RDA = 1.6 & 2.0 mg (F&M). • The vitamin, pyridoxine, is O oxidized and P O phosphorylated. O • Required for various reactions involving amino groups. pyridoxal phosphate 12 N pyridoxine (vitamin B 6) Schiff base formation H3C OH N H2N Folic Acid N N O N HN OH O O NH O O Folate • RDA = 180 & 200 mg (F&M). • Required for single carbon transfers, purine biosynthesis. • Deficiency leads to anemia. N H2N NH N OH O O NH HN NH O O O 5 13 10 tetrahydrofolate (THF or H4F) Choline HO CH2 CH2 + N H3C choline • Choline is not a vitamin or a carrier. • It is a source methyl groups and is recommended in the diet. • It has lipotropic effects due to its role in phospholipid metabolism. 14 CH3 CH3 Conversion of Pyruvate to Acetyl CoA O O O CH3 pyruvate O HO thiamine pyrophosphate (TPP) O -TPP CH3 CO2 --TPP HO CH3 lipoamide TPP • Pyruvate dehydrogenase complex O -lipoamide catalyzes these reactions, which require several vitamin-derived CH3 coenzymes. (section 6 lecture 4) CoA • Thiamine pyrophosphate is bound to + lipoamide + NAD the active site by NCI’s. + NADH • Covalently bound lipoamide is reduced when acetyl group forms. O S -CoA • Coenzyme A is a substrate. CH3 • NAD+ is required to oxidize lipoamide back to the disulfide state. 15 acetyl CoA Oral Manifestations of Avitaminosis DISEASE CLINICAL SYMPTOMS BIOCHEMICAL LESION NUTRIENT Rickets enamel hypoplasia* inadequate synthesis of calcium binding protein by intestinal cells vitamin D Beriberi magenta tongue**, angular cheilosis impaired carbohydrate metabolism; many oxidative decarboxylations cannot occur thiamine (B1) Scurvy loose teeth, bluish-red gingivitis, bleeding at gums proper collagen not made; other hydroxylations do not occur ascorbate (C) Pellagra stomatitis, glossitis**, cheilosis impaired carbohydrate metabolism and electron transport due to reduced NAD+ niacin 16 Oral Manifestations, con’t. DISEASE CLINICAL SYMPTOMS BIOCHEMICAL LESION NUTRIENT none named stomatitis, glossitis**, cheilosis Impaired carbohydrate metabolism and electron transport due to reduced FAD riboflavin (B2) none named stomatitis impaired amino acid metabolism pyridoxine (B6) Megaloblastic anemia (Sprue) stomatitis, glossitis**, cheilosis impaired DNA synthesis due to reduced thymine production folate Pernicious anemia glossitis Impaired methyl group transfers vitamin B12 *Enamel hypoplasia is caused also by vitamin A deficiency during tooth development, or by chronic renal failure-induced decrease in 1,25-dihydroxycholecalciferol production. **It is often difficult to distinguish glossitis, magenta tongue (an archaic term) or red beefy tongue as symptoms. Any one can indicate systemic problems due to iron deficiency or multiavitaminosis. 17 Food Pyramid (USDA 1996) •Heavy reliance on carbohydrates. •Less than 30% calories from fats & oils. •Aims for cardiovascular health. •Too many servings? 18 Revised 1996 Revised Food Pyramid (USDA 2005) Revised 1996 More fruits, vegetables, nuts, seeds, and legumes. Less fat, oil, sugar, and meats. Fats, oils, 2-3/wk ! Sweets, 5/week Low-fat or fat-free, 2-3 Revised 2005 For 2000 cal/day Exercise ~30 min/day Meat, poultry, fish, 2 or less Nuts, seeds, legumes, 4-5 Whole grains. 4-5 4-5 Exercise. Whole grain 7-8 19 •Study indicates reduced risks of major chronic diseases. •Healthy eating index (HEI) evaluates carbohydrates, fats and oils. •Very low in animal fats and refined starch. •High in fish and vegetable oils. •Notice exercise at base. •From McCullogh etal (2002) American Journal of Clinical Nutrition 76:1261. 20 Healthy Eating Pyramid (alternative to USDA pyramid) Fat and Heart Disease Locale Fat Calories Percent of Total Rate of Heart Disease* Japan Eastern Finland Crete 10 38 40 500 3,000 200 *Incidence of coronary heart disease per 10,000 men over ten year period. What kind of fat is more important than percent calories from fat. Willett & Stampter (2002) Scientific American 288:64-71. 21 Fast Food Analysis breakfast MCDONALD'S SAUSAGE MCMUFFIN WITH EGG Nutrient Total Rec. %Rec Calories 2676.02 2300 116.35% Pro (g) 96.37 63 152.97% Fat (g) 129.98 76.67 169.53% lunch BURGER KING DOUBLE CHEESEBURGER lunch MCDONALD'S LARGE FRENCH FRIES lunch 12 Oz CARBONATED BEVERAGE-COLA Carb (g) 333.64 dinner 3 slices medium PIZZA HUT SUPREME PAN PIZZA Na (mg) 4290.17 2400 178.76% Vit A (IU) 2084.55 5000 41.69% dinner 16 Oz CARBONATED BEVERAGE-COLA Vit C (mg) 40.08 90 44.53% Sat fat (g) 43.06 25.56 168.47% Chol (mg) 394.34 300 131.45% No snacks! 22 From http://nat.crgq.com/mainnat.html -- Next topic Extracellular Macromolecules: Glycosaminoglycans; proteoglycans; mucins