Plant Metabolism: Catabolism, Anabolism & Metabolites

• Primary metabolism involves reactions in plant cells, creating diverse molecules crucial for the plant and its environment
• Metabolism comprises coupled reactions in organism cells, including catabolism and anabolism

Catabolism

• Catabolism extracts energy from nutrients like carbohydrates and lipids through degradation

Anabolism

• Anabolism synthesizes components needed for cell structure and function

Primary Metabolites

• Primary metabolites are organic molecules in all plant cells, essential for survival
• Four primary metabolite categories: carbohydrates, lipids, amino acids, and nucleic acids

Carbohydrates

• Carbohydrates are the most abundant organic molecules in nature
• Carbohydrates are hydrates of carbon or saccharides, with the general formula Cn(H2O)n
• Polyhydroxylated carbonyl compounds (aldehyde or ketone) that appear early in photosynthesis

Carbohydrate Classification

• Classified by function: aldoses (aldehyde function) and ketoses (ketone function)
• Classified by carbon number: trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), heptoses (7C)
• Carbohydrates categorized by polymerization degree as osides
• Oligosaccharides: polymers of 2-10 oses; the most common are disaccharides
• Polysaccharides: consist of more than 10 units

Osides

• Carbohydrates in polymerized form
• Can be holosides (only oses) or heterosides (oses + non-carbohydrate part)

Carbohydrate Biosynthesis

• Synthesized during photosynthesis
• Formula: 6CO2 + 6H2O + light energy → C6H12O6 (glucose) + 6O2
• Short-formula oses are formed during the Calvin cycle and the pentose phosphate cycle
• Osides are ose polymers linked by osidic or glycosidic bonds

Calvin Cycle

• CO2 fixation in photosynthesis
• Pentose phosphate pathway involved
• Glycolysis degrades carbohydrates into pyruvate

Aerobic Conditions

• Pyruvate is transformed into acetyl CoA and oxaloacetate
• They enter the Krebs cycle

Key Plant Carbohydrates

• D-glucose is abundant in fruits (dried apricots, prunes, dates) and honey
• D-glucose is a significant energy source
• D-fructose is abundant in fruits and honey; commonly found in oligosaccharides (sucrose) and polysaccharides
• D-fructose is used in parenteral nutrition, especially for diabetics (slow intestinal absorption, no insulin secretion)
• Mannitol (polyol) is found naturally in Fraxinus ornus and brown algae; used as an osmotic diuretic and laxative
• Sorbitol (polyol) has less sweetening power than sucrose; naturally found in Sorbus aucuparia and some algae
• Sorbitol is used for its laxative and diuretic properties, and in ascorbic acid preparation

Holosides

• Sucrose (C12H22O11) is a diholoside from sugar cane and beets; linked glucose and fructose via α(1↔2)β glycosidic bond
• Sucrose is plants' main temporary energy reserve, circulating in sap and used as an excipient and sweetener

Homogeneous Reserve Polyosides

• Starch is a complex carbohydrate made of D-glucose chains
• The main glucidic reserve form in plants; found in storage organs for survival

Starch Composition

• Amylose and amylopectin are two homopolymers
• Amylose contains 600-1,000 glucose molecules (glucose-α-1,4-glucose)
• Amylopectin (isoamylose) has 10,000-100,000 glucose units; branching with alpha 1→6 bonds every 24-30 monosaccharides

Inulin

• Fructosan: a fructose polymer linked by β(2-1) to a terminal glucose molecule
• Accumulated in roots of Asteraceae, Boraginaceae, Campanulaceae, Poaceae and Alliacées families
• It is used as a dietary supplement that stimulates the growth of beneficial intestinal bacteria

Homogeneous Structural Polyosides

• Cellulose is a carbohydrate made of linear D-glucose chains linked by β-(1→4) bonds
• It is a primary plant cell wall component
• Cellulose is used as a raw fiber for paper and as a food additive

Heterogeneous Polyosides

• Pectins are plant-exclusive substances found in the middle lamella
• Pectins found in large amounts in seeds of apples, quinces, and citrus
• Act as gastrointestinal protectants, hemostatics, slow drug elimination, and as emulsifiers in ointments

Lipids

• Lipids are hydrophobic or amphipathic organic molecules characterized by at least one fatty acid
• Non-volatile and are named "fixed oils"
• Found structurally in cell membranes (phospholipids, glycolipids), coatings (cutins, suberins), and reserves (triglycerides)
• Triglycerides are the main components of plant oils
• Lipids are in seeds (oilseeds) and the pericarp of fruits (olives, avocados) and stored as oily inclusions; absent in leaves and roots
• Fatty acids consist of a carbon atom chain, ending in a carboxyl group (-COOH) at one end and a methyl group (-CH3) at the other

Plant Fatty Acids

• Usually unsaturated; carbon atoms are linked by double bonds
• Nomenclature: they are monoacids, linear, even-numbered, and either saturated or unsaturated

Lipid Structures

• Saturated Fatty Acids [CH3 -(CH2)n - COOH], the first carbon is carboxyl
• Unsaturated Fatty Acids have double bond position expressed from the carboxyl (Δ) or methyl end (ω) as omega number

Acid Examples

• Oleic acid is an 18C with one omega-9 double bond; written as C18:1 ω9 – Linoleic acid an essential C18 fatty acid with 2 double bonds (ω6,9) – Linoleic acid is found in canola, soybean, olive, sunflower, and walnut oils
• α-linolenic acid is C18:3(9,12,15) or (18:3 n-3 or ω3), found in flaxseed, canola, olive and walnut oils

Phospholipids

• They form the plasma membrane composed of two fatty acids esterified with glycerol, where glycerol's third carbon links to a phosphate group
• The phosphate group is hydrophilic and gives polarity to the molecule in an aqueous environment

Triglycerides

• Three fatty acid molecules united with one glycerol molecule
• In saturated fatty acid biosynthesis, malonyl-CoA (C3) is formed from acetyl-CoA (C2) carboxylation
• Two carbon atoms from malonyl-CoA elongate the fatty acid chain while CO2 is released

Unsaturated fatty acid biosynthesis

• It occurs by oxidation of saturated fatty acids by oxygen
• Oleate is obtained by the oxidation of stearate

anabolism

• Glycerol-P is synthesized by reduction of dihydroxyacetone-P; phosphatase releases phosphate to form glycerol

Triglyceride Biosynthesis

• Triglycerides are synthesized from glycerol-P and acyl CoA

Lipids catabolism

• Beta oxidation used in oleaginous seeds.
• Lipid reserves are hydrolyzed by lipases
• Diverse free fatty acids are degraded by beta oxidation, producing acetyl remnants
• The beta-oxidation reaction sequence occurs in four steps per cycle
• Fatty acids require (n-1) cycles for complete oxidation into n acetyl-CoA

Vegetable Oils

• Vegetable oils used in therapy and nutrition
• Almond Oil: from Prunus dulcis, Rosaceae family for skincare
• Peanut Oil: from Arachis hypogaea, Fabaceae family as an excipient (allergy risk)
• Olive Oil: from Olea europaea, Oleaceae family; bile stimulant and laxative
• Castor Oil: from Ricinus communis, Euphorbiaceae family; drastic purgative
• Sesame Oil: from Sesamum indicum, Pedaliaceae family; mild laxative and antioxidant, medicinal solvent
• Soybean Oil: from Glycine max, Fabaceae family; used in parenteral nutrition

Nitrogenous Compounds

• Proteins are polymers of amino acids; all nitrogenous substances yield amino acids upon hydrolysis
• Acid and amine functions in two amino acids react, forming a covalent peptide bond
• Amino acids are essential metabolites for structure and enzymes and also give rise to secondary metabolites

Essential Amino Acids

• Indispensable for life
• Proteinoplasts are specialized organelles in plant cells, common in seeds
• Amino acids come from the attachment of NH3 to keto acids

Three Fixation Methods

• L-amination reduction
• Glutamine pathway
• Transamination

L-amination reduction

• Global reaction: R-CO-COOH [2H] + NH3 → R-CH-[NH2]-COOH + H2O
• Mitochondrial process from keto acids of Krebs cycle

Glutamine pathway

• Glutamine synthesis from glutamate; glutamate from α-ketoglutarate

Transamination

• The most common process

Transamination Process

• It involves using a keto acid, but the amine radical is supplied by an amino acid
• Glutamate is a major NH2 donor and enables amino acid formation by transamination
• Aspartate from oxaloacetate and alanine from pyruvate are examples
• Keto acids come from the Krebs cycle
• Amino acid degradation occurs via deamination into keto acids, which enter the Krebs cycle
• This is the inverse of their biosynthesis

General Protein Info

• Plant proteins for food industry use depending on nutrition to animal sources
• Edulcorant Properties: A sweetener is a product with a sweet taste
• Thaumatin comes form Thaumatococcus danielli Family Marantaceae, a strong sweetener, sold as Talin, accepted in European Union (E957)
• Monellin from Dioscoreophyllum volkensii, family Menispermaceae, is 2000 times sweeter
• Miraculine from Synsepalum dulcificum L. Sapotaceae, switches acid flavors to sweet sensations with 437 AA

Lectins

• Proteins/glycoproteins bind specifically/reversibly to cell membrane osidic residues not enzymatic activity
• Primarily located in the seeds

Therapeutic Properties of Lectins

• Ricin binds lectins inhibit metastases, ricinus communis
• Viscumin binds lectins tumor inhibition, Album L
• Trichosantine binds lectins acts as abortive, kirilowi
• Luffaculine and VHL, Cucurbitaceae acts as Abortive

Enzymes

• Enzymes are proteins that catalyze.

Vegetable Enzyme Properties

• Papain is used for digestive aid
• Bromelain is anti inflamitory
• Ficine is used for constipation