Properties

Understanding the physical and chemical properties of vitamins and nutrients is crucial for comprehending their absorption, transport, storage, and biological activities

General Properties of Vitamins

• Organic Compounds
  • Vitamins are organic compounds that are essential for human health
• Dietary Requirements
  • They are required in small amounts from the diet because the body cannot synthesize them adequately
• Classification
  • Vitamins are classified into two groups based on their solubility: water-soluble and fat-soluble

Water-Soluble Vitamins

• General Characteristics
  • Soluble in water
  • Absorbed directly into the bloodstream
  • Not stored in the body to a significant extent
  • Excess is excreted in the urine
  • Less risk of toxicity compared to fat-soluble vitamins
  • Act as coenzymes in various metabolic reactions

Thiamin (Vitamin B1)

• Chemical Name: Thiamin
• Structure: Contains a pyrimidine ring and a thiazole ring connected by a methylene bridge
• Solubility: Soluble in water
• Stability: Unstable to heat, especially in alkaline solutions
• Active Form: Thiamin pyrophosphate (\(TPP\))
• Biochemical Functions
  • Coenzyme for pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, and transketolase
  • Involved in carbohydrate metabolism and energy production

Riboflavin (Vitamin B2)

• Chemical Names: Riboflavin
• Structure: Consists of a flavin ring system attached to a ribitol (sugar alcohol)
• Solubility: Slightly soluble in water
• Stability: Unstable to light, especially in alkaline solutions
• Active Forms: Flavin mononucleotide (\(FMN\)) and flavin adenine dinucleotide (\(FAD\))
• Biochemical Functions
  • Coenzymes for oxidoreductase enzymes
  • Involved in energy metabolism, electron transport, and antioxidant defense

Niacin (Vitamin B3)

• Chemical Names: Nicotinic acid, nicotinamide
• Structure: Pyridine derivative
• Solubility: Soluble in water
• Stability: Stable to heat, light, and air
• Active Forms: Nicotinamide adenine dinucleotide (\(NAD^+\)) and nicotinamide adenine dinucleotide phosphate (\(NADP^+\))
• Biochemical Functions
  • Coenzymes for numerous dehydrogenase enzymes
  • Involved in energy metabolism, DNA repair, and cell signaling

Pantothenic Acid (Vitamin B5)

• Chemical Name: Pantothenic acid
• Structure: Consists of β-alanine and pantoic acid linked by a peptide bond
• Solubility: Soluble in water
• Stability: Unstable to heat, especially in acidic or alkaline solutions
• Active Form: Coenzyme A (\(CoA\))
• Biochemical Functions
  • Component of coenzyme A (\(CoA\))
  • Involved in fatty acid synthesis and oxidation, citric acid cycle, and synthesis of cholesterol and steroid hormones

Pyridoxine (Vitamin B6)

• Chemical Names: Pyridoxine, pyridoxal, pyridoxamine
• Structure: Pyridine derivative with different functional groups
• Solubility: Soluble in water
• Stability: Unstable to heat, light, and alkaline solutions
• Active Form: Pyridoxal phosphate (\(PLP\))
• Biochemical Functions
  • Coenzyme for numerous enzymes involved in amino acid metabolism
  • Involved in neurotransmitter synthesis, heme synthesis, and glycogen metabolism

Biotin (Vitamin B7)

• Chemical Name: Biotin
• Structure: Consists of a ureido ring fused with a tetrahydrothiophene ring
• Solubility: Slightly soluble in water
• Stability: Stable to heat and air, but unstable to strong acids and bases
• Active Form: Biotin is covalently bound to carboxylase enzymes
• Biochemical Functions
  • Coenzyme for carboxylation enzymes
  • Involved in fatty acid synthesis, gluconeogenesis, and amino acid catabolism

Folate (Vitamin B9)

• Chemical Names: Folic acid, pteroylglutamic acid
• Structure: Consists of a pteridine ring, p-aminobenzoic acid (\(PABA\)), and glutamic acid
• Solubility: Slightly soluble in water
• Stability: Unstable to heat, light, and acidic solutions
• Active Form: Tetrahydrofolate (\(THF\))
• Biochemical Functions
  • Coenzyme for one-carbon transfer reactions
  • Involved in DNA synthesis, amino acid metabolism, and homocysteine metabolism

Cobalamin (Vitamin B12)

• Chemical Names: Cobalamin
• Structure: Contains a corrin ring system with a central cobalt atom
• Solubility: Soluble in water
• Stability: Stable to heat, but unstable to light, strong acids, and bases
• Active Forms: Methylcobalamin and adenosylcobalamin
• Biochemical Functions
  • Coenzyme for methionine synthase and methylmalonyl-CoA mutase
  • Involved in homocysteine metabolism, DNA synthesis, and nerve function

Ascorbic Acid (Vitamin C)

• Chemical Name: Ascorbic acid
• Structure: A six-carbon lactone ring
• Solubility: Soluble in water
• Stability: Unstable to heat, light, and air; easily oxidized
• Active Form: Ascorbic acid
• Biochemical Functions
  • Antioxidant
  • Required for collagen synthesis, carnitine synthesis, and neurotransmitter synthesis
  • Enhances iron absorption

Fat-Soluble Vitamins

• General Characteristics
  • Soluble in lipids
  • Absorbed along with dietary fats
  • Stored in the liver and adipose tissue
  • Risk of toxicity with excessive intake
  • Involved in various physiological functions, such as vision, bone metabolism, and antioxidant defense

Vitamin A

• Chemical Names: Retinol, retinal, retinoic acid, beta-carotene (provitamin)
• Structure: Consists of a beta-ionone ring and an isoprenoid chain
• Solubility: Soluble in lipids
• Stability: Unstable to heat, light, and air
• Active Forms: Retinol, retinal, and retinoic acid
• Biochemical Functions
  • Vision (retinal)
  • Cell differentiation and gene expression (retinoic acid)
  • Immune function

Vitamin D

• Chemical Names: Cholecalciferol (Vitamin D3), ergocalciferol (Vitamin D2)
• Structure: Steroid derivative
• Solubility: Soluble in lipids
• Stability: Stable to heat and air, but unstable to light
• Active Form: Calcitriol (1,25-dihydroxyvitamin D)
• Biochemical Functions
  • Calcium and phosphate metabolism
  • Bone health
  • Cell growth and differentiation
• Activation
  • Cholecalciferol and ergocalciferol require two hydroxylation steps for activation
    • 25-hydroxylation in the liver forms calcidiol (25-hydroxyvitamin D)
    • 1α-hydroxylation in the kidney forms calcitriol (1,25-dihydroxyvitamin D)

Vitamin E

• Chemical Names: Tocopherols, tocotrienols
• Structure: Consists of a chromanol ring and an isoprenoid side chain
• Solubility: Soluble in lipids
• Stability: Unstable to heat, light, air, and alkaline conditions
• Active Form: Alpha-tocopherol
• Biochemical Functions
  • Antioxidant
  • Protects cell membranes from lipid peroxidation

Vitamin K

• Chemical Names: Phylloquinone (Vitamin K1), menaquinone (Vitamin K2), menadione (synthetic form)
• Structure: Consists of a quinone ring and an isoprenoid side chain
• Solubility: Soluble in lipids
• Stability: Unstable to light and alkaline solutions
• Active Forms: Phylloquinone and menaquinone
• Biochemical Functions
  • Blood clotting
  • Bone metabolism
• Action
  • Acts as a coenzyme for gamma-glutamyl carboxylase, which carboxylates glutamate residues in clotting factors II, VII, IX, and X

Properties and Functions Summary

Vitamin	Solubility	Key Chemical Characteristics	Key Active Forms	Primary Biochemical Functions
Thiamin (B1)	Water	Pyrimidine and thiazole rings	Thiamin Pyrophosphate (TPP)	Carbohydrate metabolism
Riboflavin (B2)	Water	Flavin ring system	FMN, FAD	Redox reactions, energy metabolism
Niacin (B3)	Water	Pyridine derivative	NAD+, NADP+	Redox reactions, DNA repair
Pantothenic Acid (B5)	Water	β-alanine and pantoic acid	Coenzyme A (CoA)	Fatty acid metabolism, citric acid cycle
Pyridoxine (B6)	Water	Pyridine derivative	Pyridoxal Phosphate (PLP)	Amino acid metabolism, neurotransmitter synthesis
Biotin (B7)	Water	Ureido and tetrahydrothiophene rings	Biotin-Enzyme Complex	Carboxylation reactions
Folate (B9)	Water	Pteridine, PABA, and glutamate	Tetrahydrofolate (THF)	One-carbon transfer reactions, DNA synthesis
Cobalamin (B12)	Water	Corrin ring system with cobalt	Methylcobalamin, Adenosylcobalamin	Homocysteine metabolism, DNA synthesis
Ascorbic Acid (C)	Water	Lactone ring	Ascorbic Acid	Antioxidant, collagen synthesis, iron absorption
Vitamin A	Lipid	Beta-ionone ring and isoprenoid	Retinol, Retinal, Retinoic Acid	Vision, cell differentiation, immune function
Vitamin D	Lipid	Steroid derivative	Calcitriol (1,25-(OH)2D)	Calcium and phosphate metabolism, bone health
Vitamin E	Lipid	Chromanol ring and isoprenoid	Alpha-Tocopherol	Antioxidant, cell membrane protection
Vitamin K	Lipid	Quinone ring and isoprenoid	Phylloquinone, Menaquinone	Blood clotting, bone metabolism

Clinical Significance

• Understanding of these Properties Aids In
  • Predicting the absorption, transport, and storage of vitamins within the body
  • Designing appropriate dietary recommendations and supplementation strategies
  • Developing analytical methods for measuring vitamin levels in biological samples
  • Explaining the mechanisms of vitamin deficiencies and toxicities

Key Terms

• Vitamins: Essential organic compounds required in small amounts for human health
• Water-Soluble Vitamins: Vitamins that are soluble in water and readily excreted
• Fat-Soluble Vitamins: Vitamins that are soluble in lipids and stored in the body
• Coenzyme: A substance that is required for the activity of an enzyme
• Antioxidant: A substance that protects cells from damage caused by free radicals
• Absorption: The process by which nutrients are taken up from the digestive tract into the bloodstream
• Transport: The process by which nutrients are carried in the bloodstream to tissues and organs
• Storage: The process by which nutrients are stored in the body
• Excretion: The process by which waste products are eliminated from the body
• Deficiency: A condition in which the body does not have enough of a particular nutrient
• Toxicity: A condition in which the body has too much of a particular nutrient
• Retinoids: A class of chemical compounds that are related to vitamin A
• Tocopherols: A class of chemical compounds that are related to vitamin E
• Quinones: A class of chemical compounds that are related to vitamin K