Comprehensive Biochemistry Guide: Human Vitamins, Coenzymes, and Metabolic Functions

Overview of Vitamins and Enzyme Cofactors

• Definition and Essentiality:

  • A vitamin is an organic compound required in small amounts for the proper functioning of the human body.

  • Vitamins are essential nutrients that must be obtained from dietary sources because the human body cannot synthesize them in sufficient quantities (or at all).

  • A well-balanced diet typically satisfies all standard vitamin requirements for healthy individuals.

• Role in Conjugated Enzymes:

  • Many active enzymes in human metabolism are conjugated enzymes, consisting of two structural components:

  • Apoenzyme: The purely protein component of the conjugated enzyme.

  • Cofactor: The nonprotein component necessary for biological activity.

  • Vitamins function as organic cofactors (coenzymes or cofactor precursors) within conjugated enzymes.

  • When converted into active coenzymes, vitamins bind to apoenzymes to form functional holoenzymes, enabling catalytic transformation of substrates into products.

  • Without the precursor vitamin, coenzymes cannot be synthesized, rendering the apoenzyme nonfunctional and giving rise to deficiency symptoms.

• Dietary Requirements and Supplementation:

  • Ingestion requirements vary widely across different vitamins and are measured in specific units (e.g., milligrams, micrograms).

  • Inadequate intake leads to specific physiological deficiency symptoms and clinical diseases.

  • Excessive consumption, particularly via high-dose dietary supplements, can result in hypervitaminosis and toxicity.

  • Vitamin supplementation is medically indicated under increased physiological demands, such as during pregnancy or recovery from illness.

• Synthetic vs. Natural Vitamins:

  • Laboratory-manufactured synthetic vitamins are chemically identical to vitamins naturally occurring in whole food sources.

  • The human body processes synthetic and natural vitamins identically and derives equivalent biological benefits from both.

• Classification of Vitamins:

  • There are $13$ known human vitamins, categorized into two primary chemical classes based on solubility:

  • Water-Soluble Vitamins ($9$ total): Vitamin C and the $8$ B-complex vitamins.

  • Fat-Soluble Vitamins ($4$ total): Vitamins A, D, E, and K.

• General Properties Comparison (Water-Soluble vs. Fat-Soluble):

  • Absorption: Water-soluble vitamins are absorbed directly into the bloodstream; fat-soluble vitamins enter the lymphatic system first before systemic circulation.

  • Transport: Water-soluble vitamins travel freely in blood plasma without carriers; fat-soluble vitamins often require specific protein carriers for transport.

  • Storage: Water-soluble vitamins circulate freely in fluid-filled body compartments with minimal tissue storage; fat-soluble vitamins reside in cell structures and adipose storage tissues.

  • Excretion: Excess water-soluble vitamins are excreted in urine by the kidneys; fat-soluble vitamins tend to remain sequestered in fat-storage sites.

  • Toxicity Risk: Water-soluble vitamins rarely reach toxic concentrations from dietary sources or supplements; fat-soluble vitamins present a higher toxicity risk when consumed in excess via supplements.

  • Dosage Frequency: Water-soluble vitamins require frequent, regular intake; fat-soluble vitamins can be consumed in periodic doses due to systemic storage reserves.

  • Coenzyme Relationship: Water-soluble vitamins function directly as coenzyme precursors; fat-soluble vitamins do not act as coenzymes.

Water-Soluble Vitamins

• General Characteristics:

  • Includes Vitamin C and the $8$ members of the B-vitamin complex.

  • Function directly as structural components of coenzymes in essential metabolic pathways.

Vitamin C (Ascorbic Acid)

• Biosynthesis and Species Variance:

  • Synthesized from glucose derivatives in all plant species and most animal species.

  • Humans, non-human primates, fruit bats, and guinea pigs lack L-gulonolactone oxidase and cannot biosynthesize ascorbic acid, making dietary intake essential.

• Chemical Structure and Redox Chemistry:

  • Exists in a dynamic redox equilibrium between its reduced form (L-ascorbic acid) and its oxidized form (L-dehydroascorbic acid).

  • Oxidation involves the loss of two hydrogen atoms ($2\text{H}^+ + 2\text{e}^-$), converting two adjacent hydroxyl ($\text{-OH}$) groups into carbonyl ($\text{=O}$) groups across an enediol double bond.

• Biochemical Functions:

  • Collagen Synthesis: Functions as an essential cosubstrate for prolyl and lysyl hydroxylase enzymes during post-translational modification in collagen synthesis (critical for skin, tendons, blood vessels, and ligaments).

  • General Antioxidant Activity: Acts as a water-soluble electron donor, regenerating metalloenzymes by reducing oxidized active-site metal ions (e.g., converting inactive $\text{Fe}^{3+}$ back to active $\text{Fe}^{2+}$).

  • Co-Antioxidant Synergy: Regenerates the active antioxidant form of Vitamin E (tocopherol) and maintains Folate (Vitamin $\text{B}_9$) in its reduced active form.

  • Neurotransmitter Synthesis: Acts as an obligate cofactor in dopamine $\beta$-hydroxylase for the biosynthesis of norepinephrine and catecholamine neurotransmitters.

• Quantitative Dietary Sources (Content per Serving):

  • Yellow Bell Pepper (1 medium): $341\text{ mg}$

  • Broccoli (1 cup): $132\text{ mg}$

  • Green Bell Pepper (1 cup chopped): $120\text{ mg}$

  • Papaya (1 small): $96\text{ mg}$

  • Strawberries (1 cup): $85\text{ mg}$

  • Oranges (1 large): $82\text{ mg}$

  • Kale (1 cup): $80\text{ mg}$

  • Pineapple (1 cup): $79\text{ mg}$

  • Brussels Sprouts (1 cup): $74\text{ mg}$

  • Kiwi (1 medium): $64\text{ mg}$

  • Tomatoes (1 cup): $55\text{ mg}$

  • Mango (1 cup): $22\text{ mg}$

• Recommended Dietary Intakes:

  • Adult Men: $90\text{ mg/day}$

  • Adult Women: $75\text{ mg/day}$

  • Upper Tolerable Intake Limit (UL) for Adults: $2000\text{ mg/day}$

• Deficiency Conditions and Pathologies:

  • Subclinical Manifestations: Small-cell (microcytic) anemia, formation of atherosclerotic plaques, bone fragility, impaired wound healing, and progressive muscle degeneration.

  • Clinical Disease (Scurvy): Characterized by breakdown of connective tissue, swollen and bleeding gums, mucosal hemorrhages, loose teeth, delayed wound healing, and tender, swollen joints.

The B-Complex Vitamins Overview

• Functional Role:

  • The B vitamins are metabolic precursors for primary coenzymes involved in energy transduction, catabolism, and macromolecular biosynthesis.

  • Coenzymes act as temporary carriers of specific chemical groups, electrons, or hydrogen atoms during enzyme-catalyzed reactions.

• Comprehensive B-Vitamin Coenzyme Mapping:

  • Vitamin $\text{B}_1$ (Thiamin): Coenzyme form is Thiamin pyrophosphate (TPP).

  • Vitamin $\text{B}_2$ (Riboflavin): Coenzyme forms are Flavin adenine dinucleotide (FAD / $\text{FADH}_2$) and Flavin mononucleotide (FMN / $\text{FMNH}_2$).

  • Vitamin $\text{B}_3$ (Niacin): Coenzyme forms are Nicotinamide adenine dinucleotide ($\text{NAD}^+$ / NADH) and Nicotinamide adenine dinucleotide phosphate ($\text{NADP}^+$ / NADPH).

  • Vitamin $\text{B}_5$ (Pantothenic acid): Coenzyme form is Coenzyme A (CoA).

  • Vitamin $\text{B}_6$ (Pyridoxine/Pyridoxal/Pyridoxamine): Coenzyme forms are Pyridoxal phosphate (PLP) and Pyridoxamine phosphate (PMP).

  • Vitamin $\text{B}_7$ (Biotin): Coenzyme form is Biotin (covalently attached as biocytin).

  • Vitamin $\text{B}_9$ (Folate): Coenzyme forms are Tetrahydrofolate (THF) and Dihydrofolate (DHF).

  • Vitamin $\text{B}_{12}$ (Cobalamin): Coenzyme forms are Methylcobalamin and 5'-deoxyadenosylcobalamin.

• Central Metabolic Pathways Integration:

  • Carbohydrate Metabolism:

  • Glycogen $ ightleftharpoons$ Glucose: Requires Pyridoxal phosphate (PLP).

  • Glucose $ ightarrow$ Pyruvate (Glycolysis): Requires $\text{NAD}^+$.

  • Pyruvate $ ightarrow$ Acetyl-CoA: Requires Pyruvate Dehydrogenase Complex using TPP, FAD, $\text{NAD}^+$, and CoA.

  • Tricarboxylic Acid (TCA) Cycle:

  • Utilizes $\text{NAD}^+$, TPP, FAD, and CoA at key oxidative decarboxylation steps.

  • Electron Transport Chain (ETC):

  • Complex I uses FMN and $\text{NAD}^+$; Complex II uses FAD to transfer high-energy electrons for ATP synthesis.

  • Lipid Metabolism:

  • Acetyl-CoA $ ightleftharpoons$ Fatty Acids: Requires Biotin, $\text{NADP}^+$, $\text{NAD}^+$, and FAD.

  • Amino Acid & One-Carbon Metabolism:

  • Interconversion of amino acids uses PLP, $\text{NAD}^+$, THF, and Vitamin $\text{B}_{12}$.

Vitamin B1 (Thiamin)

• Chemical Structure and Conversion:

  • Consists of a substituted pyrimidine ring linked via a methylene bridge to a thiazole ring.

  • The free form is thiamin; phosphorylation yields the active coenzyme thiamin pyrophosphate (TPP).

• Biochemical Function:

  • Serves as a coenzyme in oxidative decarboxylation reactions and the transfer of carbonyl/carbon dioxide groups.

  • Essential for energy production in the pyruvate dehydrogenase and $\text{α}$-ketoglutarate dehydrogenase reactions.

• Dietary Sources:

  • Whole-grain breads, beans, nuts, milk, peas, pork, and rice bran.

• Deficiency Pathology (Beriberi):

  • Early Manifestations: Nausea, severe fatigue, peripheral paresthesia, loss of knee and foot reflexes, muscle tenderness, and weakness.

  • Dry Beriberi: Characterized by severe emaciation, peripheral neuropathy, loss of reflexes, foot drop, wrist drop, and aphonia (vocal cord paralysis due to vagus nerve damage).

  • Wet Beriberi: Characterized by cardiovascular dysfunction, peripheral edema, dyspnea, orthopnea, right heart enlargement, and high-output heart failure.

  • Wernicke-Korsakoff Syndrome: Associated with severe thiamin deficiency in chronic alcoholism, leading to ophthalmoplegia, ataxia, acute confusion, coma, and death.

Vitamin B2 (Riboflavin)

• Chemical Structure and Coenzymes:

  • Composed of an isoalloxazine tricyclic ring system attached to a ribitol sugar alcohol side chain.

  • Forms two central redox coenzymes: Flavin Mononucleotide (FMN) and Flavin Adenine Dinucleotide (FAD).

• Biochemical Function:

  • Serves as a two-electron/two-proton hydrogen transporter in oxidation-reduction reactions across central metabolic pathways.

• Dietary Sources:

  • Milk, organ meats, eggs, dark green leafy vegetables, enriched breads, beans, and peas.

• Deficiency Pathology:

  • Causes generalized dermatitis, cheilosis (cracking of lips), angular stomatitis, glossitis, and skin lesions.

Vitamin B3 (Niacin)

• Chemical Forms and Coenzymes:

  • Occurs in two vitamer forms: Nicotinic acid (containing a carboxyl group) and Nicotinamide (containing an amide group).

  • Synthesizes two active hydride-carrying coenzymes: Nicotinamide Adenine Dinucleotide ($\text{NAD}^+$ / NADH) and Nicotinamide Adenine Dinucleotide Phosphate ($\text{NADP}^+$ / NADPH).

• Biochemical Function:

  • Functions as a reversible hydride ($\text{H}^-$) ion carrier in catabolic dehydrogenation reactions ($\text{NAD}^+$) and reductive anabolic pathways ($\text{NADP}^+$).

• Dietary Sources:

  • Lean meats, whole grains, poultry, fish, and legumes.

• Deficiency Pathology (Pellagra):

  • Characterized clinically by the classic "4 Ds": Dermatitis, Diarrhea, Dementia, and Death.

  • Symptoms include muscle weakness, loss of appetite, symmetrical photosynthetic dermatitis (Casal's necklace), and gastrointestinal mucosal inflammation.

Vitamin B5 (Pantothenic Acid)

• Chemical Structure:

  • Synthesized from an amide linkage between $\beta$-alanine and pantoic acid.

  • Forms the structural backbone of Coenzyme A (CoA) and acyl carrier protein (ACP).

• Biochemical Function:

  • Acts as a carrier for acyl groups via a high-energy thioester bond at its terminal sulfhydryl group.

• Dietary Sources:

  • Widely distributed in all plant and animal tissues, notably organ meats, nuts, and whole-grain cereals.

• Deficiency Pathology:

  • Rare due to widespread availability; extreme deficiency leads to metabolic disturbances and anemia.

Vitamin B6 (Pyridoxine)

• Chemical Forms:

  • Encompasses three free vitamer precursor compounds based on a pyridine ring:

  • Pyridoxine (PN): Alcohol form ($\text{-CH}_2\text{OH}$).

  • Pyridoxal (PL): Aldehyde form ($\text{-CHO}$).

  • Pyridoxamine (PM): Aminomethyl form ($\text{-CH}_2\text{NH}_2$).

  • Phosphorylation produces the active coenzymes: Pyridoxine-5'-phosphate (PNP), Pyridoxal-5'-phosphate (PLP), and Pyridoxamine-5'-phosphate (PMP).

• Biochemical Function:

  • Serves as a coenzyme in amino acid metabolism, including transamination, decarboxylation, and racemization, as well as glycogen phosphorylase activity.

• Dietary Sources:

  • Meats, whole grains, poultry, fish, and nuts.

• Deficiency Pathology:

  • Causes seborrheic dermatitis, peripheral neuropathy, microcytic anemia, and central nervous system disorders (e.g., convulsions).

Vitamin B7 (Biotin)

• Chemical Structure and Mechanism:

  • Bicyclic heterocyclic ring compound (tetrahydrothiophene fused to an imidazolidone ring) attached to a valeric acid side chain.

• Biochemical Function:

  • Functions as a coenzyme in ATP-dependent carboxylation reactions, transporting carbon dioxide ($\text{CO}_2$) or carboxyl groups in lipogenesis (acetyl-CoA carboxylase) and gluconeogenesis (pyruvate carboxylase).

• Dietary Sources:

  • Egg yolks, liver, yeast, nuts, and intestinal bacterial synthesis.

• Deficiency Pathology:

  • Causes dermatitis, alopecia, muscle weakness, and neurological impairment.

Vitamin B9 (Folate / Folic Acid)

• Chemical Forms:

  • Folic acid consists of a pteridine ring, para-aminobenzoic acid (PABA), and glutamic acid residues.

  • Reduced intracellularly by dihydrofolate reductase to active coenzyme forms: Dihydrofolate (DHF) and Tetrahydrofolate (THF).

• Biochemical Function:

  • Acts as a carrier of one-carbon units (methyl, methylene, methenyl, formyl) required for purine, pyrimidine, and amino acid biosynthesis.

• Dietary Sources:

  • Leafy green vegetables (foliage), asparagus, peas, and beans.

• Deficiency Pathology:

  • Causes megaloblastic (macrocytic) anemia due to impaired DNA replication and cell division, as well as neural tube defects (e.g., spina bifida) in fetal development.

Vitamin B12 (Cobalamin)

• Chemical Structure:

  • Complex corrin ring system surrounding a central cobalt ion ($\text{Co}^+$).

  • The free commercial form is Cyanocobalamin; main biological coenzyme forms are Methylcobalamin and 5'-deoxyadenosylcobalamin.

• Biochemical Function:

  • Coenzyme for methionine synthase (methyl group transfer) and methylmalonyl-CoA mutase (hydrogen atom transfer and rearrangement).

• Dietary Sources:

  • Exclusively found in animal products including meat, fish, eggs, and milk.

• Absorption and Deficiency Pathology:

  • Requires gastric Intrinsic Factor (IF) for receptor-mediated intestinal absorption in the ileum.

  • Pernicious Anemia: An autoimmune destruction of gastric parietal cells leading to intrinsic factor deficiency, preventing Vitamin $\text{B}_{12}$ absorption.

  • Clinical Manifestations: Megaloblastic anemia combined with irreversible neurological damage (subacute combined degeneration of the spinal cord).

Fat-Soluble Vitamins

• General Characteristics:

  • Includes Vitamins A, D, E, and K.

  • Hydrophobic, nonpolar molecules stored in cell membranes, liver, and adipose tissues.

  • Do not act as coenzymes in metabolic reactions.

Vitamin A

• Chemical Forms (Retinoids and Carotenoids):

  • Retinoids (Preformed Vitamin A):

  • Retinol: Alcohol form ($\text{R} = \text{-CH}_2\text{OH}$).

  • Retinal: Aldehyde form ($\text{R} = \text{-CHO}$).

  • Retinoic Acid: Carboxylic acid form ($\text{R} = \text{-COOH}$).

  • Provitamin A Carotenoids:

  • Plant pigments such as $\beta$-carotene.

  • Central enzymatic cleavage of one $\beta$-carotene molecule yields two molecules of retinal.

• Major Biochemical Functions:

  • Vision: 11-cis-retinal binds to the apoprotein opsin to form the visual pigment rhodopsin in rod cells. Absorption of light isomerizes retinal to all-trans-retinal, triggering a conformational change that initiates optic nerve impulses. Bleached rhodopsin regenerates over time; insufficient Vitamin A slows regeneration, causing night blindness.

  • Cell Differentiation: Retinoic acid acts like a steroid hormone, binding nuclear receptor proteins that modulate gene transcription, controlling stem cell differentiation into specialized tissues.

  • Epithelial Tissue Maintenance: Regulates differentiation of mucus-secreting epithelial cells. Deficiency causes keratinization, leading to dry, hard, scaly surfaces.

  • Reproduction and Growth: Supports spermatogenesis in males and fetal development in females.

• Dietary Sources:

  • Preformed: Eggs, butter, cheese, liver.

  • Provitamin: Dark green leafy vegetables and deep orange vegetables (carrots, sweet potatoes).

• Deficiency Pathology:

  • Xerophthalmia (dryness of cornea/conjunctiva), inflamed eye membranes, night blindness (nyctalopia), hyperkeratosis, and increased susceptibility to infection.

Vitamin D

• Chemical Forms and Endogenous Synthesis:

  • Vitamin $\text{D}_2$ (Ergocalciferol): Plant-derived sterol form.

  • Vitamin $\text{D}_3$ (Cholecalciferol): Animal-derived form, also synthesized endogenously in human skin.

  • Endogenous Pathway: Ultraviolet (UV) light converts 7-dehydrocholesterol (a normal cholesterol metabolite in the epidermis) into Cholecalciferol.

• Biochemical Function:

  • Regulates systemic calcium ($\text{Ca}^{2+}$) and phosphate ion homeostasis.

  • Hydroxylated in the liver and kidneys to form 1,25-dihydroxycholecalciferol (calcitriol), which:

  • Stimulates intestinal absorption of calcium and phosphate ions.

  • Decreases renal excretion of calcium and phosphate.

  • Stimulates osteoblast synthesis of calcium-binding proteins to promote bone mineralization.

• Dietary Sources:

  • Fatty fish-liver oils, fortified milk, and egg yolks.

• Deficiency Pathology:

  • Rickets (Children): Inadequate bone mineralization causing soft, weak bones and skeletal deformities, including bowlegs (Varus deformity) or knock-knees (Valgus deformity).

  • Osteomalacia (Adults): Demineralization of existing bone matrix leading to bone pain, muscle weakness, and increased fracture risk.

  • Hypocalcemic Tetany: Involuntary muscle spasms from low blood calcium levels.

Vitamin E

• Chemical Structure and Tocopherols:

  • Group of eight substituted chromanol ring derivatives with isoprenoid side chains, categorized into four tocopherol isomers ($α$-, $\beta$-, $γ$-, $δ$-tocopherol).

  • $α$-Tocopherol exhibits the highest biological activity.

• Chemical Stability:

  • Readily destroyed by exposure to oxygen, light, oxidation, and high temperatures (e.g., deep frying).

• Biochemical Function:

  • Primary lipid-soluble chain-breaking antioxidant in cell membranes.

  • Protects polyunsaturated fatty acids (PUFAs) in membrane phospholipids and Vitamin A from free radical peroxidation.

  • Neutralizes reactive oxygen species (ROS) by donating a hydrogen atom, stopping destructive free radical chain reactions.

  • Spent Vitamin E radicals are reduced and recharged by Vitamin C.

• Dietary Sources:

  • Whole-grain cereals, vegetable oils (wheat germ, sunflower, olive), margarine, nuts, and seeds.

• Deficiency Pathology:

  • Increased oxidative fragility of cell membranes, leading to erythrocyte hemolysis (hemolytic anemia) and neuromuscular degeneration.

Vitamin K

• Chemical Forms:

  • Vitamin $\text{K}_1$ (Phylloquinone): Plant form containing a phytyl side chain with a single double bond.

  • Vitamin $\text{K}_2$ (Menaquinone): Bacterial form containing an isoprenoid side chain with multiple double bonds ($n = 1\text{--}13$, predominantly $7\text{--}9$ repeats).

• Origin:

  • Approximately $50\%$ is synthesized endogenously by intestinal gut microbiota; the remaining $50\%$ is derived from dietary sources.

• Biochemical Function:

  • Essential co-substrate for $γ$-glutamyl carboxylase, which converts specific glutamic acid residues into $γ$-carboxyglutamate (Gla) residues.

  • Required for post-translational modification and activation of clotting factors (Prothrombin [Factor II], Factors VII, IX, X) and bone proteins (osteocalcin).

• Dietary Sources:

  • Cabbage, potatoes, green peas, and dark leafy green vegetables (spinach, kale).

• Deficiency Pathology:

  • Defective blood coagulation leading to prolonged prothrombin time, excessive bruising, mucosal bleeding, and severe hemorrhagic disorders.

Summary Reference Table of Vitamins

• Water-Soluble Class Summary:

  • $\text{B}_1$ (Thiamin): Sources = bread, beans, nuts, milk, peas, pork, rice bran; Function = coenzyme in decarboxylation reactions; Deficiency = Beriberi (nausea, severe exhaustion, paralysis).

  • $\text{B}_2$ (Riboflavin): Sources = milk, meat, eggs, dark green vegetables, bread, beans, peas; Function = forms FMN and FAD hydrogen transporters; Deficiency = Dermatitis.

  • Niacin ($\text{B}_3$): Sources = meat, whole grains, poultry, fish; Function = forms $\text{NAD}^+$ hydride transporter; Deficiency = Pellagra (weak muscles, loss of appetite, diarrhea, dermatitis).

  • $\text{B}_6$ (Pyridoxine): Sources = meat, whole grains, poultry, fish; Function = carries amino and carboxyl groups; Deficiency = Dermatitis, nervous disorders.

  • $\text{B}_{12}$ (Cobalamin): Sources = meat, fish, eggs, milk; Function = coenzyme in amino acid metabolism; Deficiency = Pernicious anemia.

  • Folic Acid ($\text{B}_9$): Sources = leafy green vegetables, peas, beans; Function = coenzyme in methyl group transfers; Deficiency = Anemia.

  • Pantothenic Acid ($\text{B}_5$): Sources = all plants and animals, nuts, whole grains; Function = part of Coenzyme A acyl carrier; Deficiency = Anemia.

  • Biotin ($\text{B}_7$): Sources = egg yolk, liver, yeast, nuts; Function = coenzyme in fatty acid synthesis; Deficiency = Dermatitis, muscle weakness.

  • Vitamin C (Ascorbic Acid): Sources = citrus fruits, tomatoes, green peppers, strawberries, leafy greens; Function = synthesis of collagen for connective tissue; Deficiency = Scurvy (tender tissues, weak bleeding gums, swollen joints).

• Fat-Soluble Class Summary:

  • Vitamin A (Retinol): Sources = eggs, butter, cheese, dark green and deep orange vegetables; Function = synthesis of visual pigments; Deficiency = Inflamed eye membranes, night blindness, skin scaliness.

  • Vitamin D (Calciferol): Sources = fish-liver oils, fortified milk; Function = regulation of calcium and phosphorus metabolism; Deficiency = Rickets (bone malformation).

  • Vitamin E (Tocopherol): Sources = whole-grain cereals, margarine, vegetable oils; Function = prevention of oxidation of Vitamin A and membrane fatty acids; Deficiency = Red blood cell breakage.

  • Vitamin K: Sources = cabbage, potatoes, peas, leafy green vegetables; Function = synthesis of blood-clotting proteins; Deficiency = Blood-clotting disorders.