Biochemistry: Vitamins & Cofactors

General Properties of Vitamins and Cofactors

• Definition: A vitamin is an organic or organometallic compound essential for the proper functioning and metabolic homeostasis of the human body.

• Dietary Requirement: Because the human body cannot synthesize vitamins in adequate amounts (with minor exceptions), they must be obtained through dietary sources.

• Dosage: Vitamins are required in microgram ($\mu\text{g}$) and milligram ($\text{mg}$) quantities daily.

• Natural vs. Synthetic: Natural and synthetic vitamins possess identical chemical structures and biological functions.

• Functional Role: Many enzyme cofactors are derived directly from vitamins. Deficiency in a vitamin leads to cofactor deficiency, which renders enzymes inactive and results in specific disease states.

• Supplementation: Supplemental vitamins may be required following illness or specific clinical conditions.

• Classification: Vitamins are divided into two main chemical classes:

  • Water-Soluble Vitamins: B-complex vitamins and Vitamin C (Ascorbic acid).

  • Fat-Soluble Vitamins: Vitamins A, D, E, and K.

Cofactor-Assisted Enzyme Catalysis

• Apoenzyme Activation: An inactive enzyme active site requires binding of a specific vitamin derivative (coenzyme) to form a catalytically active holoenzyme.

• Catalytic Cycle:

  1. The coenzyme binds to the active site of the enzyme.

  2. The substrate enters the active site and interacts with both the enzyme and coenzyme.

  3. Chemical reaction occurs, converting substrate into product(s).

  4. The product(s) and coenzyme are released or recycled for subsequent catalytic cycles.

Water-Soluble Vitamins and Coenzymes

• Water-soluble vitamins act as precursors for activated carriers and coenzymes in metabolism.

Vitamin B1 (Thiamine)

• Coenzyme Form: Thiamine Pyrophosphate ($\text{TPP}$).

• Enzymatic Conversion: Thiamine is converted to TPP by the enzyme TPP synthetase.

• Metabolic Function: Acts as a coenzyme in carbohydrate metabolism, specifically catalyzing the synthesis or cleavage of bonds adjacent to carbonyl carbons (aldehyde transfer reactions).

• Deficiency Disease: Beriberi (characterized by weight loss, heart problems, and neurological dysfunction).

Vitamin B2 (Riboflavin)

• Coenzyme Forms: Flavin Mononucleotide ($\text{FMN}$) and Flavin Adenine Dinucleotide ($\text{FAD}$).

• Metabolic Function: Functions as a prosthetic group in oxidation-reduction (redox) reactions, carrying electrons (e.g., within the pyruvate dehydrogenase complex).

• Deficiency Symptoms: Ariboflavinosis, cheliosis, angular stomatitis (lesions of the mouth and lips), and dermatitis.

Vitamin B3 (Niacin / Nicotinic Acid)

• Chemical Forms: Nicotinic acid contains a carboxyl group ($\text{-COOH}$), whereas Nicotinamide contains an amide group ($\text{-CONH}_2$).

• Coenzyme Forms: Nicotinamide Adenine Dinucleotide ($\text{NAD}^+$) and Nicotinamide Adenine Dinucleotide Phosphate ($\text{NADP}^+$).

• Endogenous Biosynthesis: The liver can synthesize Niacin from the amino acid Tryptophan; however, this biosynthetic pathway requires Riboflavin ($\text{Vitamin B}_2$), Pyridoxine ($\text{Vitamin B}_6$), and Iron ($\text{Fe}$).

• Metabolic Function: Serves as an electron acceptor/donor in oxidation-reduction reactions.

• Redox Mechanism: The nicotinamide ring accepts a hydride ion ($\text{H}^-$):

$\text{Nicotinamide ring} + 2[\text{H}^\cdot] \rightleftharpoons \text{Reduced ring} + \text{H}^+$

• Example Reaction (Alcohol Dehydrogenase, ADH):

$\text{CH}_3\text{CH}_2\text{OH} + \text{NAD}^+ \rightleftharpoons \text{CH}_3\text{CHO} + \text{NADH} + \text{H}^+$

• Protein Context: Glyceraldehyde-3-phosphate dehydrogenase (GAPDH, PDBid 1GD1) is a 2-domain protein subunit of mass $37\,\text{kDa}$ that binds $\text{NAD}^+$ to catalyze glycolytic redox reactions.

• Deficiency Disease: Pellagra (characterized by dermatitis, depression, and diarrhea).

Vitamin B5 (Pantothenic Acid)

• Coenzyme Form: Coenzyme A ($\text{CoA}$).

• Metabolic Function: Acyl-group transfer reactions.

• Deficiency Symptoms: Paresthesia and hypertension.

Vitamin B6 (Pyridoxine)

• Coenzyme Form: Pyridoxal 5'-phosphate ($\text{PLP}$).

• Chemical Structural Change: Pyridoxal ($\text{PL}$) possesses an aldehyde group ($\text{-CHO}$) and an alcohol group ($\text{-CH}_2\text{OH}$). Phosphorylation replaces the hydroxyl hydrogen to form a phosphate ester ($\text{-CH}_2\text{-O-PO}_3^{2-}$), yielding the active coenzyme PLP.

• Metabolic Function: Group transfer reactions to or from amino acids; essential for amino acid synthesis and transamination.

• Deficiency Symptoms: Depression, confusion, convulsions, and various neurological conditions.

Vitamin B7 (Biotin)

• Coenzyme Form: Biotin covalently attached to a lysine residue of a carrier protein (Biotin-lysine adducts / biocytin).

• Metabolic Function: ATP-dependent carboxylation and carboxyl-group transfer.

• Deficiency Symptoms: Rash about the eyebrows, muscle pain, and fatigue (deficiency is extremely rare).

Vitamin B9 (Folic Acid)

• Coenzyme Form: Tetrahydrofolate ($\text{THF}$).

• Metabolic Function: Transfer of one-carbon units; thymine nucleotide synthesis.

• Deficiency Symptoms: Anemia, birth defects, and developmental neural-tube defects.

Vitamin B12 (Cyanocobalamin / Cobalamin)

• Coenzyme Forms: Adenosylcobalamin and Methylcobalamin ($5'$\text{-Deoxyadenosylcobalamin}).

• Chemical Structure: An organometallic complex containing a central cobalt ion ($\text{Co}^+$ or $\text{Co}^{3+}$) coordinated within a corrin ring structure, bound to a cyano group ($\text{-CN}$) in cyanocobalamin or a methyl group ($\text{-CH}_3$) in methylcobalamin.

• Metabolic Function: Transfer of methyl groups and intramolecular rearrangements.

• Deficiency Disease: Pernicious anemia, general anemia, and methylmalonic acidosis.

Vitamin C (Ascorbic Acid)

• Coenzyme / Functional Role: Coenzyme in the hydroxylation reactions required for the formation of the structural protein collagen; antioxidant; involved in the metabolism of specific amino acids.

• Tissue Saturation and Excretion: A daily dose of $100\,\text{mg/day}$ completely saturates all body tissues. Any excess intake above saturation is excreted in urine.

• Deficiency Disease: Scurvy (manifested by swollen and bleeding gums, subdermal hemorrhaging, and impaired tissue healing). A Treatise on the Scurvy was published in 1751 documenting its cause and cure.

Choline: An Essential Water-Soluble Nutrient

• Classification: A water-soluble essential nutrient grouped alongside the B-complex vitamins.

• Chemical Structure: A quaternary ammonium cation ($[(\text{CH}_3)_3\text{N}^+\text{CH}_2\text{CH}_2\text{OH}]\text{X}^-$).

• Biological Function: Forms the polar head group of major membrane phospholipids essential for biological membrane architecture:

  • Phosphatidylcholine: A structural phospholipid found in the plasma membranes of all non-neuronal cells.

  • Sphingomyelin: A structural phospholipid component specifically enriched in the membranes of nerve and brain cells.

Fat-Soluble Vitamins (Non-Coenzyme Roles)

• General Characteristics: Hydrocarbon-like structures containing few polar functional groups; nonpolar and insoluble in water; involved in plasma membrane processes; do not function as enzyme coenzymes.

Vitamin A (Retinol)

• Precursor: Derived from dietary $\beta$\text{-carotene}.

• Active Forms: Three primary retinoid forms exist in the body:

  • Retinol ($\text{R} = \text{-CH}_2\text{OH}$)

  • Retinal ($\text{R} = \text{-CHO}$)

  • Retinoic acid ($\text{R} = \text{-COOH}$)

• Biological Function: Critical roles in vision phototransduction, cell growth, and reproduction.

• Deficiency Symptoms: Night blindness, cornea damage, and structural damage to respiratory and gastrointestinal epithelial tracts.

Vitamin D (Calciferol)

• Biosynthesis: Vitamin $\text{D}_3$ (Cholecalciferol) is generated in the skin by the action of ultraviolet (UV) light on 7-dehydrocholesterol.

• Active Metabolite: $1,25$\text{-dihydroxycholecalciferol} ($1,25$\text{-dihydroxyvitamin D}_3) represents the most biologically active form.

• Biological Function: Regulation of calcium homeostasis (promotes bone mineralization) and phosphate metabolism.

• Deficiency Conditions:

  • Rickets (in children): Skeletal deformities and impaired growth.

  • Osteomalacia (in adults): Soft, bending, weakened bones.

Vitamin E (\alpha-Tocopherol)

• Chemical Structure: Contains a chromanol ring with a phenolic hydroxyl group ($\text{-OH}$) and a long hydrophobic isoprenoid side chain.

• Antioxidant Mechanism: The phenolic hydroxyl group donates a hydrogen atom to quench free radicals. The hydrophobic tail allows the molecule to intercalate and diffuse freely within lipid bilayers, protecting membranes against lipid peroxidation.

• Deficiency Symptoms: Lesions in muscles and nerves (rare).

Vitamin K

• Chemical Forms: A group of related compounds including phylloquinone ($\text{K}_1$) and menaquinones ($\text{K}_2$).

• Biological Function: Blood coagulation; serves as an essential factor for post-translational $\gamma$\text{-carboxylation} of glutamate residues required to convert preprothrombin into active prothrombin.

• Clinical Note: Unlike many other vitamins, vitamin K is not typically ingested as a general dietary supplement.

• Deficiency Symptoms: Subdermal hemorrhaging and prolonged blood clotting times.

Essential Metal Ion Cofactors (Metalloenzymes)

• Metal ions are required in minute quantities and function as essential trace elements or micronutrients for enzyme activity.

• Metals and Associated Enzymes:

  • Iron ($\text{Fe}^{2+}$ and $\text{Fe}^{3+}$): Catalase, peroxidase, aconitase, and cytochrome oxidase.

  • Zinc ($\text{Zn}^{2+}$): Alcohol dehydrogenase, carboxypeptidase A, carboxypeptidase B, DNA polymerase, and carbonic anhydrase.

  • Copper ($\text{Cu}^{2+}$): Cytochrome oxidase, lysyl oxidase, ascorbate oxidase, and superoxide dismutase.

  • Magnesium ($\text{Mg}^{2+}$): Hexokinase and glucose-6-phosphatase.

  • Manganese ($\text{Mn}^{2+}$): Arginase.

  • Potassium ($\text{K}^+$): Pyruvate kinase.

  • Nickel ($\text{Ni}^{2+}$): Urease.

  • Molybdenum ($\text{Mo}^{4+}$ and $\text{Mo}^{6+}$): Nitrate reductase.

  • Cadmium ($\text{Cd}^{2+}$): Carbonic anhydrase (in conjunction with $\text{Zn}^{2+}$).

Reference Summary Tables

B-Complex Vitamins

• Thiamine ($\text{B}_1$) | Coenzyme: Thiamine pyrophosphate | Reaction: Aldehyde transfer | Deficiency: Beriberi (weight loss, heart problems, neurological dysfunction)

• Riboflavin ($\text{B}_2$) | Coenzyme: Flavin adenine dinucleotide ($\text{FAD}$) | Reaction: Oxidation-reduction | Deficiency: Cheliosis, angular stomatitis (mouth lesions), dermatitis

• Pyridoxine ($\text{B}_6$) | Coenzyme: Pyridoxal phosphate ($\text{PLP}$) | Reaction: Group transfer to or from amino acids | Deficiency: Depression, confusion, convulsions

• Nicotinic acid (Niacin, $\text{B}_3$) | Coenzyme: Nicotinamide adenine dinucleotide ($\text{NAD}^+$) | Reaction: Oxidation-reduction | Deficiency: Pellagra (dermatitis, depression, diarrhea)

• Pantothenic acid ($\text{B}_5$) | Coenzyme: Coenzyme A | Reaction: Acyl-group transfer | Deficiency: Hypertension, paresthesia

• Biotin ($\text{B}_7$) | Coenzyme: Biotin-lysine adducts | Reaction: ATP-dependent carboxylation and carboxyl-group transfer | Deficiency: Rash about eyebrows, muscle pain, fatigue (rare)

• Folic acid ($\text{B}_9$) | Coenzyme: Tetrahydrofolate | Reaction: Transfer of one-carbon components; thymine synthesis | Deficiency: Anemia, neural-tube defects in development

• Cobalamin ($\text{B}_{12}$) | Coenzyme: $5'$\text{-Deoxyadenosylcobalamin} | Reaction: Transfer of methyl groups; intramolecular rearrangements | Deficiency: Anemia, pernicious anemia, methylmalonic acidosis

Noncoenzyme Vitamins

• Vitamin A | Function: Vision, growth, reproduction | Deficiency: Night blindness, cornea damage, damage to respiratory and gastrointestinal tracts

• Vitamin C (Ascorbate) | Function: Antioxidant, collagen synthesis | Deficiency: Scurvy (swollen and bleeding gums, subdermal hemorrhaging)

• Vitamin D | Function: Regulation of calcium and phosphate metabolism | Deficiency: Rickets (children: skeletal deformities, impaired growth), Osteomalacia (adults: soft, bending bones)

• Vitamin E | Function: Antioxidant | Deficiency: Lesions in muscles and nerves (rare)

• Vitamin K | Function: Blood coagulation | Deficiency: Subdermal hemorrhaging