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Questions and Answers
What is a primary metabolite primarily involved in?
Secondary metabolites are primarily produced during the growth phase of an organism.
False
Name a primary metabolite commonly used in food production.
Citric acid
Ethanol and lactic acid are examples of primary ________.
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Which of the following is an example of a secondary metabolite?
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Match the following metabolites with their characteristics:
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Primary metabolites are not essential for proper growth.
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What role do secondary metabolites commonly serve in ecological functions?
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What type of receptors does atropine act upon?
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Erythromycin is produced from the organism Streptomyces erythreus.
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What is the main pigment involved in the photosynthesis process?
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During the light-dependent reactions of photosynthesis, _____ is produced from water.
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Match the following antibiotics with their sources:
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What are the components produced during the light reactions of photosynthesis?
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The Calvin cycle occurs in the chloroplast stroma.
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What is the initial molecule that CO2 combines with during carbon fixation?
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The metabolic pathway that converts glucose into pyruvate is known as _____ .
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Match the following stages of the Calvin cycle with their functions:
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Which of the following equations represents the light reactions of photosynthesis?
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Bacitracin is primarily used as an oral antibiotic.
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What is the net gain of ATP from glycolysis?
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Which of the following is NOT a product of the shikimic acid pathway?
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The Krebs cycle occurs in the cytoplasm of the cell.
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What are the primary substrates used in the shikimic acid pathway?
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The mevalonic acid pathway is responsible for the synthesis of ________.
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Match the following pathways with their primary products:
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Which intermediate is derived from acetyl-CoA in the biosynthesis of fatty acids?
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Building blocks for secondary metabolites are primarily sourced from the Krebs cycle.
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What role does chorismate play in the shikimic acid pathway?
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The enzymes involved in the shikimic acid pathway catalyze reactions that produce ________ from phosphoenol pyruvate and erythrose-4-phosphate.
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What environmental factors can trigger the expression of phenolic compounds?
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The malonic acid pathway requires the involvement of acyl carrier protein (ACP).
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What are isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) derived from?
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________ is a seven-step metabolic pathway used by various organisms for biosynthesis of folates and aromatic amino acids.
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Match the following acids with their corresponding pathway of biosynthesis.
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Which vitamins are classified as water-soluble?
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Vitamins can provide energy and build cellular structures.
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What are some common dietary sources of Vitamin A?
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Vitamin _____ is known as the sunshine vitamin.
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What vitamin deficiency leads to rickets in children?
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Match the vitamin with its main function:
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Which vitamin is associated with the prevention of night blindness?
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Fat-soluble vitamins are stored in the body.
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Name one condition caused by Vitamin D deficiency.
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The stable form of Vitamin D is known as Vitamin _____.
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Which vitamin is associated with maintaining skin health?
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Excessive intake of fat-soluble vitamins is always safe.
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What is the role of Vitamin D in the body?
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Vitamins _____ and _____ are known to support immune system function.
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Which of the following can be a source of Vitamin E?
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What is the primary use of Ouabin (G-strophanthin)?
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Which of the following vitamins is known to improve mood?
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Oleander is considered safe for any level of consumption.
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What are the components of the trisaccharide coupled to strophanthidin?
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Vitamin C is the most stable of all the vitamins.
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What is the primary role of Vitamin B7 in metabolism?
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The main toxic component found in the leaves of Nerium oleander is called ________.
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Vitamin B12 is critical in the synthesis of ______ cells.
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Match the cardiac glycosides with their corresponding plant sources:
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Which nutrient is essential for brain development and functions?
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A deficiency of Vitamin B5 can cause fatigue and sleep disturbances.
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List two sources of Vitamin B6.
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The sugar portion of glycosides is called ______.
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What is one of the uses of Vitamin C?
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Match the following vitamins with their main usage:
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Vitamin B9 is important for the production of DNA and RNA.
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What deficiency can cause peripheral neuropathy?
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Deficiency in Vitamin C leads to ______.
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What role does Vitamin B12 play in the body?
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Fatty acid synthesis is a function of Vitamin B7.
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Which of the following is a major dietary source of Vitamin E?
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Vitamin K is essential for blood clotting.
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What is the most common symptom of Vitamin K deficiency?
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Vitamin B1 is also known as ______.
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Match the vitamins with their primary function:
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What is a common dietary source of Vitamin B2?
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Vitamin B1 deficiency can lead to brain deterioration.
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What is a deficiency condition associated with Vitamin B3?
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Riboflavin is sensitive to ______.
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Match the following vitamins with their deficiency symptoms:
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Which Vitamin is important for the formation of blood vessels?
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The intestinal microflora provide a significant supply of Vitamin D.
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What role does Vitamin B3 play in metabolism?
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Vitamin B5 is known as the ______ factor.
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Which of the following vitamins is NOT water-soluble?
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Vitamin K deficiency does not impact the immune system.
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Which glycoside form is predominantly found in plants?
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The aglycone part of glycosides is responsible for their solubility properties.
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What is the role of glycosides in plants?
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Glycosides are usually insoluble in __________ solvents.
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Match the type of glycosides with their corresponding examples:
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Which enzyme is found to hydrolyze most β-glycoside linkages?
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All glycosides have the same stability profile under acidic conditions.
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What is the main pharmacological role attributed to the aglycone part of glycosides?
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Glycosides can be hydrolyzed by __________ enzymes that are typically found in the same plant.
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Match the glycoside classification with their defining features:
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Which of the following is a property of glycosides?
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All glycosides are derived from the same aglycone.
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Describe how glycosides are formed in plants.
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The hydrolytic cleavage of glycosides can be achieved by heating with a __________ acid.
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What is the primary action of cardioactive glycosides on cardiac muscle?
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Cardioactive glycosides are derived from a C21 steroid and a C2 unit.
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Name one of the principal glycosides found in Digitalis purpurea.
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The aglycones of cardioactive glycosides are referred to as __________.
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Match the following glycosides with their respective sources:
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Which of the following structures is characteristic of cardenolides?
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Bufadienolides contain a pentadienolide ring.
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What role does the Na+/K+ ATPase enzyme serve in cardiac glycoside action?
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The concentration of cardioactive glycosides in Digitalis purpurea is approximately __________.
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Which of the following modifications represent acetylated derivatives of purpurea glycosides?
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All glycosides derived from Digitalis lanata belong to the same aglycone type.
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Identify one common consequence of inhibiting the Na+/K+ ATPase enzyme.
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Match the following classifications of glycosides with their characteristics:
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The compounds referred to as __________ are derived from digoxigenin and diginatigenin.
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Study Notes
Metabolism
- Metabolism refers to all chemical transformations within living organisms' cells, essential for life.
- Metabolites are the end products and intermediates of metabolic processes.
- Primary metabolites are directly involved in growth, development, and reproduction, performing physiological functions, typically formed during the growth phase.
- Examples of primary metabolites include ethanol, lactic acid, amino acids, and citric acid.
Secondary Metabolites
- Secondary metabolites are organic compounds produced by modifying primary metabolite synthases.
- They are typically formed at the end of or near the stationary phase of growth and are not directly involved in essential physiological functions.
- Many secondary metabolites serve ecological roles, like defense mechanisms, by acting as antibiotics or producing pigments.
- Examples include atropine, erythromycin, and bacitracin.
Photosynthesis
- Photosynthesis is the process of converting light energy into stored chemical energy in simple sugar molecules.
- It occurs in chloroplasts and other green parts of plants.
- The overall equation is: 6CO2 + 12H2O + light energy -> C6H12O6 + 6O2 + 6H2O
- Photosynthesis involves two key stages: light-dependent reactions and light-independent reactions.
Light-Dependent Reactions
- Occur during daylight in the thylakoid membranes of chloroplasts.
- Grana, sac-like structures within the thylakoid, gather light and contain photosystems.
- Photosystems are protein and pigment complexes that capture light energy.
- Two types of photosystems exist: photosystem I and photosystem II.
- Light energy is converted into ATP and NADPH, which are used in the subsequent light-independent reactions.
- Water is used, and oxygen is produced during this process.
- Chemical equation for the light reaction: 2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2NADPH + 3ATP
Light-Independent Reactions
- Also known as the Calvin cycle or dark reactions.
- Occur in the stroma of the chloroplast, utilizing ATP and NADPH from the light reactions.
- Plants capture carbon dioxide through stomata and incorporate it into glucose using the Calvin cycle.
- ATP and NADPH drive the conversion of 6 CO2 molecules into one glucose molecule.
- Chemical equation for the dark reaction: 3CO2 + 6 NADPH + 5H2O + 9ATP → G3P + 2H+ + 6 NADP+ + 9 ADP + 8 Pi (G3P - glyceraldehyde-3-phosphate)
Calvin Cycle Reactions
- Divided into three main stages: carbon fixation, reduction, and regeneration of RuBP.
- Carbon fixation: CO2 combines with ribulose 1,5-bisphosphate (RuBP), catalyzed by rubisco, forming a 6-carbon compound that splits into two 3-carbon molecules of 3-phosphoglycerate (PGA).
- Reduction: ATP and NADPH convert PGA into glyceraldehyde-3-phosphate (G3P).
- Regeneration: Some G3P is used to make glucose, while the rest is recycled to regenerate RuBP, requiring ATP and a complex series of reactions.
Glycolysis
- Converts glucose into pyruvate in the cytosol.
- Occurs independently of oxygen.
- Free energy released generates a net gain of two ATP molecules.
Citric Acid Cycle (Krebs cycle)
- A series of enzyme-catalyzed reactions in the mitochondrial matrix that oxidizes acetyl-CoA to form carbon dioxide and reduce coenzymes.
- Reduced coenzymes generate ATP in the electron transport chain.
- An eight-step process.
- Occurs in the mitochondrial matrix under aerobic conditions.
Biosynthesis of Secondary Metabolites
- Biosynthesis is a multi-step, enzyme-catalyzed process that converts substrates into more complex products.
- Simple compounds are modified, converted, or joined together to form macromolecules.
- Building blocks are small molecules derived from primary metabolites, particularly from photosynthesis, glycolysis, and the Krebs cycle.
- Important building blocks include acetyl CoA, shikimic acid, mevalonic acid, and malonic acid.
- Building blocks are categorized based on their carbon unit count:
- C1 (derived from S-methyl of L-methionine)
- C2 (derived from acetyl-CoA)
- C5 (derived from isoprene units)
- C6-C3 (derived from phenylalanine or tyrosine through the shikimic acid pathway)
Pathways for Secondary Metabolites Synthesis
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Shikimic Acid Pathway: The basis for biosynthesis of phenolic compounds, alkaloids, and others, occurring in plant chloroplasts.
- Aromatic compounds produced through this pathway are abundant in plants and are triggered by stress like pathogens, herbivores, pH changes, temperature, UV radiation, salinity, and heavy metals.
- Uses phosphoenol pyruvate and erythrose-4-phosphate as starting materials to produce chorismate, a precursor for aromatic amino acids (phenylalanine, tyrosine, and tryptophan).
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Malonic Acid Pathway: Also known as the acetate/malonate pathway, it involves acyl carrier protein (ACP) to generate fatty acylthioesters of ACP.
- These acyl thioesters serve as intermediates in fatty acid synthesis.
- C2 acetyl CoA units produce even-numbered fatty acids.
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Mevalonic Acid Pathway: Also known as the isoprenoid pathway, it involves the synthesis of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR).
- Essential for cholesterol biosynthesis, steroid hormones, myelin sheets in the nervous system, vitamin D precursors, and synaptic vesicle formation and release.
- The pathway produces isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), which act as building blocks for isoprenoids.
- These building blocks yield geranyl pyrophosphate (C10-monoterpenes), farnesyl pyrophosphate (C15-sesquiterpenes), and geranylgeranyl pyrophosphate (C20-diterpenes).
- Farnesyl pyrophosphate creates squalene, which forms cholesterol and other triterpenoids.
Vitamin E
- Commonly found in plant oils, green vegetables, whole grains, egg yolks, and meats
- Wheat germ oil is a traditional natural source
- Protects cell membranes, proteins, and DNA from oxidation, contributing to cellular health
- Prevents oxidation of unsaturated fatty acids and lipids in cells
- Supports immune function
- Deficiency can cause nerve and muscle damage, leading to loss of feeling in limbs, movement control, muscle weakness, and vision problems
- Deficiency indicates a weakened immune system
Vitamin K
- Refers to 2-methyl-1,4-naphthoquinone and its derivatives
- Widely distributed in dairy products, fruits, and vegetables, with green leafy vegetables being particularly rich sources
- Intestinal microflora contributes significantly to the body's supply
- Primarily acts in blood clotting (antihemorrhagic activity)
- Used to treat bleeding events caused by warfarin overdose
- Involved in bone protein metabolism (osteocalcin)
- Without Vitamin K, osteocalcin cannot bind to bone-forming minerals, resulting in poor bone mineralization
- Regulates blood calcium levels
- Deficiency causes hemorrhage
Vitamin B1 (Thiamine)
- Consists of pyrimidine and thiazole rings linked by a methylene bridge
- Commercially prepared by chemical synthesis, usually used as the hydrochloride salt
- Stable in acidic environments but decomposes quickly above pH 5.0
- Approximately 50% of the vitamin in foods is destroyed during cooking
- Good dietary sources are whole grains, legumes, and meats
- Alcohol inhibits absorption
- Participates in energy metabolism, converting carbohydrates, lipids, and proteins into energy
- Crucial for nerve and muscle activity
- May be beneficial for individuals with Alzheimer's disease
- Supports myelin sheath development and improves brain function
- Deficiency can lead to Wernicke encephalopathy and beriberi
- Risk factors include alcohol dependence, malabsorption, and a thiamine-deficient diet
Vitamin B2 (Riboflavin)
- Yellow, heat-stable substance with limited water solubility
- Sensitive to light, transforming into lumichrome or lumiflavin depending on the acidity of the solution
- Neither lumichrome nor lumiflavin possess biological activity
- Yeast is the richest natural source
- Primary dietary sources include dairy products, eggs, legumes, and meats
- Small amounts are found in cereal grains, fruits, and green vegetables
- Stable during cooking in the absence of light
- Occurs in foods as free riboflavin, riboflavin 5'-phosphate (FMN), and flavin adenine dinucleotide (FAD)
- Nucleotides are hydrolyzed to riboflavin in the upper gastrointestinal tract
- Free riboflavin is readily absorbed by intestinal mucosal cells through an active transport system enhanced by bile salts
- Acts as a coenzyme in numerous oxidation-reduction reactions, essential for energy release from carbohydrates, fats, and proteins
- Stimulates growth and reproduction
- Plays a role in vision
- Contributes to the conversion of B6, folic acid, and niacin into their active coenzyme forms
- Neutralizes free radicals, acting as an antioxidant
- Deficiency leads to stomatitis and dermatitis
Vitamin B3 (Niacin)
- Naturally occurring pyridine derivative that prevents pellagra
- Niacinamide (nicotinamide) also occurs naturally, possesses anti-pellagra activity, and is used for dietary and therapeutic purposes
- Readily absorbed from the gastrointestinal tract under normal conditions
- Good dietary sources include meats, fish, and dairy products
- Coffee bean roasting releases a significant quantity of niacin and contributes to its characteristic flavor
- Tryptophan is converted to niacin in the body
- Acts as a coenzyme in energy-transfer reactions
- Similar to riboflavin coenzymes, it carries hydrogen during metabolic reactions
- Protects against neurological degeneration and Alzheimer's disease
- Helps lower LDL cholesterol
- Lowers the risk of cardiovascular diseases and alleviates arthritis
- Deficiency causes pellagra, characterized by symptoms affecting the nervous system, skin, and gastrointestinal tract. These are often summarized as the "3D's" - dementia, dermatitis, and diarrhea.
- Oral lesions, such as angular stomatitis and red tongue, are more distinctive than other symptoms.
Vitamin B5 (Pantothenic Acid)
- Component of the vitamin B complex, sometimes called the "chick antidermatitis factor"
- Naturally occurring compound that yields β-alanine and pantoic acid (a substituted butyric acid derivative) upon hydrolysis
- Rich dietary sources include animal organs (heart, kidney, liver) and cereal grains
- Turns food into energy
- Involved in the synthesis of lipids, neurotransmitters, steroid hormones, and hemoglobin
- Maintains and repairs skin and hair tissues and cells
- Helps heal wounds and lesions
- Normalizes blood lipid profile
- Deficiency causes fatigue and sleep disturbances
Vitamin B6
- Term encompassing pyridoxol, pyridoxal, and pyridoxamine: three closely related, naturally occurring, highly substituted pyridine derivatives with comparable biological activity
- Pyridoxine is commonly used to refer to pyridoxol in pharmacy and medicine. It is the predominant form in plant materials.
- Pyridoxal and pyridoxamine occur in animal tissues.
- Pyridoxine, the most stable form, is typically used for exogenous dietary supplementation and therapeutic purposes due to its stability.
- Rich sources include beef liver, tuna, salmon, fortified cereals, chickpeas, poultry, and certain vegetables and fruits, especially dark leafy greens, pineapple, papaya, oranges, and cantaloupe.
- Improves mood
- Essential for biological reactions involving amino acid metabolism, neurotransmitter synthesis, and red blood cell formation
- Acts as a critical cofactor in a wide range of biochemical reactions regulating cellular metabolism
- Deficiency causes peripheral neuropathy
- Symptoms of Vitamin B6 deficiency resemble those of niacin and riboflavin deficiencies, including neurological abnormalities, skin lesions, and hypochromic microcytic anemia.
Vitamin B7 (Biotin)
- Most bioavailable sources include eggs, fish, meat, seeds, nuts, and certain vegetables like sweet potatoes
- Plays a role in metabolism as a coenzyme that transfers carbon dioxide
- Essential for breaking down food (carbohydrates, fats, and proteins) into energy
- Involved in various cellular reactions, particularly in fat and protein metabolism of hair roots, fingernails, and skin
- Used in fatty acid synthesis
- Deficiency causes fatigue, depression, and dermatitis
Vitamin B9 (Folate)
- Also known as folacin, pteroylglutamic acid, and vitamin B9
- Rich dietary sources include beans, peanuts, sunflower seeds, fresh fruits, fruit juices, whole grains, and liver
- Essential for brain development and function
- Aids in the production of DNA and RNA
- Involved in the metabolism of vitamins and amino acids
- Crucial during early pregnancy to reduce the risk of birth defects of the brain and spine
- Required for the synthesis of glycine, methionine, and nucleotides T & U
- Deficiency can lead to megaloblastic and macrocytic anemias and glossitis
Vitamin B12 (Cobalamins)
- Series of porphyrin-related corrinoid derivatives that function as extrinsic factors to prevent pernicious anemia
- Cyanocobalamin, a red crystalline material, is the most stable cobalamin and is commonly used in therapy
- Hydroxocobalamin, where the cyano group is replaced by a hydroxyl substituent, also finds therapeutic use
- Acts as a coenzyme converting homocysteine to methionine
- Plays a role in the metabolism of fatty acids and amino acids
- Contributes to neurotransmitter production
- Maintains the protective lining surrounding nerve fibers
- Bone cell activity depends on Vitamin B12
- Plays a significant role in DNA synthesis
- Supports brain function and red blood cell synthesis
- Deficiency usually affects rapidly dividing cells in the hematopoietic system (e.g., megaloblastic anemia) and can cause irreversible neurological damage (e.g., defective myelin nerve sheaths). Common symptoms include irritability, weakness, memory loss, mood swings, and tingling or numbness in the arms and legs.
Vitamin C (Ascorbic Acid)
- Naturally occurring vitamin that prevents scurvy and possesses antioxidant properties
- Exists in equilibrium with dehydro-L-ascorbic acid, an oxidized form, which also has antiscorbutic properties
- The least stable of all vitamins
- Good dietary sources include citrus fruits, tomatoes, strawberries, and other fresh fruits and vegetables
- Vitamin C content is preserved by freezing, but up to 50% is lost during cooking
- One of its key properties is its antioxidant activity
- Functions in enzyme activation and oxidative stress reduction
- Plays a role in collagen synthesis and iron absorption
- Protects against infections and inflammation
- Helps prevent certain diseases such as cancer, the common cold, cardiovascular diseases, and cataracts
- Deficiency causes scurvy
Glycosides
- Organic compounds, often of plant origin, composed of a sugar portion (glycon) linked to a non-sugar moiety (aglycon or genin) by a glycosidic bond.
- Four main classes: C-glycosides, O-glycosides, S-glycosides, and N-glycosides
- Glycosides yield sugars upon enzymatic or acid hydrolysis
- Sugar components can be mono-, di-, tri-, or tetrasaccharides
- Sugars exist in isomeric α and β forms, with β-form more prevalent in plants
- Chemically, glycosides are acetals where the glycon hydroxyl group condenses with the aglycone hydroxyl group
- Cleaved in the body to glycon and aglycon components
- Glycon: Contributes to solubility, impacting absorption and distribution
- Aglycon: Responsible for pharmacological activity
- No generalizable stability due to their complex structure
- Solubility properties are variable
- Most glycosides are soluble in water or hydroalcoholic solutions but insoluble or less soluble in non-polar organic solvents due to the sugar moiety increasing water solubility
- Aglycon is soluble in non-polar organic solvents like benzene, ether, and chloroform.
- Hydrolyzed by heating with dilute acid or appropriate enzymes, often found in the same plant, in separate compartments.
- A specific enzyme is typically responsible for hydrolyzing each glycoside.
- The same enzyme can hydrolyze different glycosides, but α and β isomers are usually not hydrolyzed by the same enzyme. For example, emulsin hydrolyzes most β-glycosides while maltase and invertase, which are α-glycosidases, are capable of hydrolyzing α-glycosides.
- Glycosides are generally solid, amorphous, non-volatile, odorless, and often bitter, with the exception of flavonoids (yellow) and anthraquinones (red or orange)
- Play important roles in plant life, serving as:
- Sugar reserves
- Waste products of plant metabolism
- Means of detoxification
- Regulators of osmosis
- Regulators of metabolically important substances supply
- Defenses against microbial invasion
- Some aglycones are antiseptic and bactericidal.
- Many therapeutic agents are derived from glycosides, contributing to various therapeutic classes
- Cardiac glycosides from digitalis, strophanthus, squill, and others
- Laxative drugs like senna, aloe, rhubarb, cascara sagrada, and frangula containing emodin and other anthraquinone glycosides
- Sinigrin, a glycoside from black mustard, yields allyl isothiocyanate, a potent local irritant.
Classification of Glycosides
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According to glycosidic linkage:
- α-glycosides (α sugar)
- β-glycosides (β sugar)
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According to the chemical group of the aglycon involved in glycoside formation:
- O-glycosides (OH group): For example, senna and rhubarb
- C-glycosides (C group): For example, cascaroside from cascara
- S-glycosides (SH group): For example, sinigrin from black mustard
- N-glycosides (NH group): For example, glycoalkaloids.
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According to the chemical nature of the aglycon:
- Cardioactive group
- Anthraquinone group
- Saponin group
- Cyanophore group
- Isothiocyanate group
- Flavonol group
- Alcohol group
- Aldehyde group
- Phenol group
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According to the nature of the simple sugar component:
- Glucoside (glycone is glucose)
- Galactoside (glycone is galactose)
- Mannoside (glycone is mannose)
- Arabinoside (glycone is arabinose)
Biosynthesis of Glycosides
- Biosynthetic pathways vary widely depending on the aglycone and glycone units
- Aglycone and sugar parts are biosynthesized separately and coupled to form a glycoside.
- Coupling involves phosphorylation of a sugar to yield sugar 1-phosphate, which then reacts with uridine triphosphate to form a uridine diphosphate sugar (UDP-sugar) and inorganic phosphate.
- UDP-sugar reacts with the aglycone to form the glycoside and a free UDP.
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Summary of the reaction steps:
- sugar phosphorylation sugar-1-P
- UTP + sugar-1-P UDP-sugar + Ppi
- UDP-sugar + aglycone sugar-aglycone + UDP (glycoside)
Extraction of Glycosides
- Since glycosides are accompanied by specific hydrolyzing enzymes, these enzymes must be inactivated, which can be achieved by:
- Boiling the plant in water or alcohol
- Defatting or purification of plant material, particularly seeds
- Treatment with lead acetate to precipitate tannins and other non-glycosidal impurities
- Removal of excess lead acetate using hydrogen sulfide (H2S) gas.
- Filtration and concentration of the extract to yield the crude glycoside
- Purification of the crude glycosides using chromatography or crystallization
Cardioactive Glycosides
- Highly specific action on cardiac muscle, increasing tone, excitability, and contractility
- Aglycones of these glycosides are referred to as "cardiac genin."
- Steroidal in nature, specifically derivatives of cyclopentaphenanthrene with an unsaturated lactone ring attached to C17.
- Two types of genin:
- Cardenolides: Contain a five-membered lactone ring. Examples include digitoxigenin (23 carbons) and glycosides from digitalis and strophanthus species.
- Bufadienolides (Scilladienolides): Contain a six-membered lactone ring. Examples include scillarenin (24 carbons) and the glycosides of squill and Bufotoxin
- Cardenolides have a butenolide (4 carbons) at C17, also called an α,β-unsaturated lactone ring.
- Scilladienolides have a pentadienolide (5 carbons with two double bonds) at C17, also called a pentenolide.
- These two types of lactone rings react differently with certain color tests.
- The glycosides of digitalis and strophanthus species have a characteristic structure with R= CH3.
- Glycosides of strophanthus have an aldehyde or alcohol group at C17, represented by R = CHO or CH2OH.
Bufadienolides
- Bufadienolides are a class of cardiac glycosides.
- Squill glycosides contain R1=OH, R2=H.
- Bufotoxin contains R1 & R2 = ester group.
- Cardiac glycosides can contain up to four monosaccharide molecules linked in series.
Structure of Cardiac Glycosides
- Cardiac glycosides contain a beta-OH group at position C-3 which is always involved in a glycosidic linkage.
- Cardiac glycosides contain another beta-OH group at C-14.
- Cardiac glycosides contain an unsaturated 5 or 6-membered lactone ring at position C-17, also in the β configuration.
- Cardiac glycosides may have additional OH groups at C-5, C-11 and C-16.
- The nomenclature of cardiac glycosides involves arranging functional groups, denoting configuration, type of glycoside, and position of double bonds.
- If a compound has one double bond it is called a cardenolide, if it has two double bonds it is called a dienolide, if it has no double bond it is called a cardanolide or bufanolide.
Biosynthesis of Cardiac Glycosides
- Cardiac glycosides are derived from mevalonic acid.
- The aglycones of cardiac glycosides are derived from mevalonic acid.
- The final molecules arise from a condensation of a C21 steroid with a C2 unit for C-22 and C-23.
- Bufadienolides are condensation products of a C21 steroid and a C3 unit.
Drugs Containing Cardioactive Glycosides
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Digitalis or Foxglove:
- The dried leaf of Digitalis purpurea (Scrophulariaceae).
- Digitalis is from the Latin "digitus" meaning finger, referring to the finger shaped corolla.
- Purpurea is Latin and refers to the purple colour of the flower.
- Contains a large number of glycosides, the most important being digitoxin, gitoxin, and gitaloxin.
- Average concentration is about 0.16%.
- Contains other glycosides like purpurea glycosides A and B, gluco-gitaloxin, and gluco-digitoxigenin.
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Digitalis lanata:
- Contains nearly 70 different glycosides.
- All are derivatives of five different aglycones, three of which (digitoxigenin, gitoxigenin, and gitaloxigenin) also occur in D. purpurea.
- The other two types of glycosides are derived from digoxigenin and diginatigenin, occurring in D. lanata but not D. purpurea.
- Leaves are used as a source of digoxin and lanatoside C.
- Lanatoside A, B, and E are acetyl derivatives of purpurea A, B, and E respectively.
- Glycosides derived from Digitoxigenin:
- Lanatoside A = Digitoxigenin---DX---DX----DX(AC)---G
- Acetyl-digitoxin = Digitoxigenin---DX---DX----DX---(AC)
- Digitoxin = Digitoxigenin------DX---DX----DX
- Purpurea gly A = Digitoxigenin---DX---DX----DX---G
- DX = Digitoxose, DX (AC)=Acetyldigitoxose,G = Glucose.
Mechanism of Action
- Digitalis acts by inhibiting Na+/K+ ATPase enzyme.
- This enzyme maintains K+ inside the cell and Na+ outside the cell.
- Inhibition causes a block in K+ transport back into the cell, increasing its concentration in the extracellular fluid.
- At the same time, Na+ ions enter the cell, promoting the entry of Ca +2 which is essential for muscle contraction.
- These agents are used in the treatment of congestive heart failure.
Strophanthus
- Dried ripe seeds of Strophanthus kombe or Strophanthus hispidus (Apocyanaceae).
- Principal glycosides are K-strophanthoside, K-strophanthin-B and cymarin, all based on the genin strophanthidin.
- Constituents:
- K-strophanthoside, also known as strophoside.
- The main glycoside in both S. kombe and S. hispidus.
- Composed of the genin strophanthidin coupled to a trisaccharide consisting of cymarose, β-glucose, and α-glucose.
- Used intravenously (IV) as a cardiotonic.
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Ouabin (G-strophanthin)
- Obtained from S. grantus (Apocynaceae).
- Used as a cardiotonic, often given intravenously for prompt therapeutic effect.
- Absorption from the alimentary canal is slow and irregular, therefore oral administration is not recommended and considered unsafe.
- Ouabagenin differs from K-strophanthidin in having 2 additional (OH) groups at C-1 and C-11 and having an alcoholic group at C-10 instead of the aldehydic group.
Oleander
- Leaves of Nerium oleander (Apocynaceae) have been used to treat cardiac insufficiency
- Main constituent is oleanderin, a promising agent for anticancer treatment.
- Oleander has historically been considered a poisonous plant due to the toxicity of some of its compounds, especially to animals when consumed in large amounts.
Squill
- The white bulb of Urginea maritima (Mediterranean squill) or Urginea indica (Indian squill), F: Liliaceae.
- Genins of squill glycosides differ from cardenolides in:
- They have a six-membered doubly unsaturated lactone ring in position C-17.
- They have at least one double bond in the steroid nucleus.
- Uses: as an expectorant but also possesses emetic, cardiotonic and diuretic properties.
- Red squill, consisting of the bulb of the red variety of Urginea maritima, is mostly used as rat poison.
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