Carbohydrates: Structure, Types, Digestion and Metabolism

• Carbohydrates are structures, types, the processes of digestion, absorption, and metabolism.
• They are the major dietary source of energy for animals.
• Animal metabolism produces energy, which is the reverse of photosynthesis in plants.
• They comprise 60% to 90% of plant tissue and are the basic energy source in animal cells.
• Plant cells use chlorophyll to trap solar energy, producing carbohydrates and oxygen.
• In plant cells, carbohydrates can be found as sugar, starch, or cellulose.
• When animals consume plants, the carbohydrates are broken down to produce energy, the opposite of photosynthesis.

Structure and Classification of Carbohydrates

• Carbohydrates are classified based on the number of carbon atoms, into five groups and by other classifications.
• Triose (3 C), tetrose (4 C), pentose (5 C), and hexose (6 C) are based on the number of carbon atoms.
• The "ose" suffix in a biochemical name indicates that the molecule is a sugar.
• Monosaccharides are also known as simple sugars.
• Disaccharides contain two monosaccharide units.
• Oligosaccharides contain 3-10 monosaccharide units.
• Polysaccharides contain more than 10 monosaccharide units.
• Conjugated carbohydrates are covalently bound to lipids or proteins, forming glycolipids or glycoproteins.

Monosaccharides

• They are the simplest form of carbohydrates, often called simple sugars, which cannot be hydrolyzed into simpler compounds.
• Monosaccharides are divided into aldoses (polyhydroxy aldehydes) and ketoses (polyhydroxy ketones) depending on containing an aldehyde or ketone group.
• Most nutritionally important sugars are pentoses or hexoses.
• Common monosaccharides important in animal nutrition:glucose, fructose (levulose), galactose, mannose, and ribose.
• Glucose and fructose have the same molecular formula, C6H12O6, and are hexoses, but glucose is an aldose, and fructose is a ketose.
• Glucose, fructose, and galactose are the three nutritionally and metabolically important hexoses.
• A monosaccharide with a free aldehyde group or a free ketone group is a reducing sugar, reducing another compound and is highly soluble in water.
• Monosaccharides present in plant cell walls are hexoses (-glucose, -galactose, and mannose), pentoses (-arabinose and -xylose), 6-deoxyhexoses (-rhamnose and -fucose), and uronic acids (glucuronic and galacturonic or their 4-0-methyl ethers).
• Nutritionally important sugars are of the D-form.
• Starch is a soft polymer consisting of a-D-glucose units, a pliable substance readily digested.
• Isomers are chemical compounds that have identical chemical formulae but differ in properties and the arrangement of atoms in the molecule.
• D- and L-glucose differ in their orientation of hydroxyl groups on carbon 5.
• Cellulose is the most abundant organic compound but cannot be digested by autoenzymatic digesters because they do not produce the enzyme and contain rigid polymers with β-D-glucose.
• Glucose, formerly known as grape sugar, is the carbohydrate circulating in blood (blood sugar) and used by the body for energy production, and can exist as a and ẞ isomers.
• Fructose, a common monosaccharide, acts as an alternative metabolite, providing energy to metabolic pathways like glycogen, triglyceride synthesis, free fatty acid synthesis, and gluconeogenesis.
• Galactose is a common monosaccharide and part of lactose in milk and used to make sulfatide, which helps with immune response and nervous system signaling in galactan, a plant polymer.

Disaccharides

• Nutritionally important disaccharides include:
• Sucrose (glucose + fructose, e.g., table sugar).
• Lactose (glucose + galactose, milk sugar).
• Maltose (a-D-Glucose + ẞ-D-Glucose, malt sugar).
• Cellobiose (ẞ-D-Glucose + β-D-Glucose, cellulose).
• They are made up of two monosaccharides bonded together by a glycosidic (covalent) bond, with the elimination of one water molecule.
• Sucrose is formed from one molecule of -D-glucose and one molecule of ß-D-fructose, with no active reducing group transported in plants in high concentrations in sugar cane (200 g/kg) and sugar beet (150-200 g/kg); easily hydrolyzed by the enzyme sucrase or by dilute acids.
• Maltose (malt sugar) is produced during the hydrolysis of starch and glycogen by dilute acids or enzymes from starch during barley germination by amylase and is water-soluble but not as sweet as sucrose.
• Cellobiose does not exist naturally and is the basic repeating unit of cellulose by two -D-glucose residues linked through a β-(1:4)-bond and is split only by microbial enzymes and has one active reducing group.
• Lactose (milk sugar) is a product of the mammary gland, with cow's milk containing 43-48 g/kg also less soluble and sweet than sucrose and is formed from one molecule of β-D-glucose joined to one of β-D-galactose in a β-(1:4)-linkage and is fermented by Streptococcus lactis, which sours milk by converting lactose into lactic acid (CH3.CHOH.COOH).

Oligosaccharides

• They are made by bonding together three or more (3 to 15) monosaccharides bonded together.
• Raffinose (glucose + fructose + galactose; 3 sugars)
• Stachyose (glucose + fructose + 2 galactose; 4 sugars)
• In animal diets, oligosaccharides are commonly found in beans and legumes.
• Some are used as substances to enhance the growth of good microbes (prebiotics).
• There has been an increased interest in using different oligosaccharides as feed additives to enhance hindgut health (e.g., fructooligosaccharides, mannan oligosaccharides).
• Short-chain oligosaccharides are not nutritionally significant for mammals because mammals do not secrete enzymes to digest them and reduce the potential digestible energy in feeds for nonruminants with microbes in the hindgut, fermenting, causing gas production and flatulence.
• Several oligosaccharides, such as mannan oligosaccharide (MOS), fructooligosaccharides (FOS), and galactooligosaccharides (GOS), are used as feed additives to modify gut microflora, stimulate the intestinal immune system, and stimulate the growth of beneficial gut microbes.

Polysaccharides

• Important homopolysaccharides in animal nutrition: starch (nonstructural form), glycogen (animal form), and cellulose (plant structural form).
• Starch is the principal sugar form of carbohydrate in cereal grains, the basic unit is a-D-Glucose and is the chief carbohydrate source in the diet in monogastric animals.
• Amylose is the simplest polysaccharide, comprised solely of glucose units joined in an alpha 1,4 linkage, is water soluble and constitutes 15% to 30% of total starch in most plants.
• Amylopectin is not water-soluble and constitutes 70% to 85% of total starch in plant cells as the major form of starch and are made by joining together large polymers of simple sugars and store energy and provide structural support, complex chains of many monosaccharide units.
• Nonstarch polysaccharides, or resistant starch, cannot be digested by animal enzymes but are fermented by hindgut and rumen microbes.
• Glycogen is a form of starch in animal tissue, called animal starch, which is related to amylopectin with basic alpha-D-Glucose but with a mix of a 1,4 and a 1,6 bonds found in a small amount (< 1%) in liver and muscle tissue.
• Cellulose is found in plant cell walls, providing structural integrity and is highly stable only microbial cellulase can degrade ruminant animals have bacteria with cellulase, which breaks down cellulose to release sugar for energy.

Digestion and Absorption

• Food's organic components are large, insoluble molecules that must be broken down into simpler compounds before absorption into the bloodstream and that this process involves mechanical, chemical, and microbial activities.
• Mechanical activities: mastication and muscular contractions.
• Chemical action: enzymes in digestive juices, but plant enzymes may also play a minor role.
• Microbial digestion: enzymic, carried out by bacteria, protozoa, and fungi, and is important in ruminant digestion.
• Carbohydrate digestion occurs in the small intestine, where pancreatic amylase digests starch granules.
• Some birds have salivary amylase in their mouths.
• The small intestine is the site of carbohydrate digestion in farm animals.
• Amylase acts in two forms: one that randomly cleaves a 1,4 bonds and the other that removes disaccharide units (maltose).
• Pancreatic amylase does not act on a 1,6 bonds that form the branch points in amylopectin, and the end products of amylase digestion include a mixture of glucose, maltose, and dextrins, acted upon by a 1,6 glucosidase.
• Monogastric animal's end product of carbohydrate digestion is mainly glucose.
• Glucose and fructose can be absorbed directly through the intestine, and starch digestion ends in the brush border.
• Disaccharides, such as maltase and isomaltase, break down disaccharides into monosaccharides with the help of enzymes on the brush border, which are then absorbed into the enterocyte.
• Sucrose becomes glucose and fructose through sucrase, and lactose turns into glucose and galactose through lactase in young animals that are still on milk, with amylase activity is low which increases when they start consuming solid foods.
• Ruminant carbohydrate digestion: mainly due to microbial fermentation in the rumen, which is anaerobic respiration.
• The VFAs produced by rumen bacteria from dietary carbohydrates serve as the animal's absorbed energy source.
• Monogastric animals digest carbohydrates to produce monosaccharides (like glucose) on the microvillus membrane surface, transported across the cell to the liver via the portal blood systems and absorbed at concentrations (galactose, glucose, fructose, mannose, xylose, and arabinose) in decreasing order of magnitude.
• Carbohydrate digestion in many animals (not ruminants) begins in the mouth with salivary amylase secretion.
• The major enzyme involved in starch digestion is pancreatic amylase.

Pathways in Carbohydrates Metabolism

• They act as energy sources and in the structural elements in living cells which focuses on synthesizing and using glucose.
• During glycolysis, an ancient pathway found in almost all organisms, a small amount of energy is captured as a glucose molecule is converted into two pyruvate molecules.
• Glycogen, a storage form of glucose in vertebrates, is synthesized by glycogenesis when glucose levels are high and degraded by glycogenolysis when glucose is in short supply.
• Noncarbohydrate precursors by reactions referred to as gluconeogenesis.

Glycolysis

• Glycolysis focuses on synthesizing and using glucose and is transported throughout the body in the blood and is degraded by the glycolytic pathway (in glycolysis, each glucose molecule is split into two three-carbon units (pyruvate) and the energy captured is stored temporarily in molecules of ATP and NADH) if energy reserves are low.
• Glucose molecules not required for immediate energy production are stored as glycogen in the liver and muscle and when ATP is not generated from the pentose phosphate pathway.
• Energy requirements of many tissues depend on flow of glucose and can also synthesize monosaccharides, fatty acids, and certain amino acids.
• Glycolysis occurs in almost every living cell, which is anaerobic and in preeukaryotic Earth, each glucose molecule is split into pyruvate (three-carbon units), with several carbon atoms oxidized and the energy stored temporarily in two molecules of ATP and NADH and the fate depends on the organism and circumstances.
• In anaerobic organisms, pyruvate may be converted to waste products such as ethanol, lactic acid, acetic acid, and similar molecules.
• The 10 reactions of the glycolytic pathway: synthesis of glucose-6-phosphate, conversion of glucose-6-phosphate to fructose-6-phosphate, the phosphorylation of fructose-6-phosphate, cleavage of fructose-1,6-bisphosphate, the interconversion of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, oxidation of glyceraldehyde-3-phosphate, phosphoryl group transfer, the interconversion of glycerate-3-phosphate, the interconversion of glycerate-3-phosphate and glycerate-2-phosphate, dehydration of glycerate-2-phosphate, and synthesis of pyruvate.
• In terms of energy, glycolysis produces two ATPs and two NADHs per glucose molecule.
• Pyruvate, can yield to most of cells convert to acetyl-CoA converted during oxidation in citric acid cycle, electron transport system produces water and the release energy.
• Some cells and organisms convert pyruvate to lower compounds and regenerate NAD+ by muscle cells, red blood cells, and certain bacterial species.
• Gluconeogenesis occurs primarily in the liver with precursor molecules including lactate, pyruvate, glycerol, and certain a-keto acids and kidney can make new glucose and brain and red blood cells rely on it exclusively.
• Lactate is released from red blood cells and other cells (Cori cycle) and converted to pyruvate by lactate dehydrogenase.
• Glycerol, the product of fat metabolism, is transported and converted to glycerol-3-phosphate by glycerol kinase.
• Alanine helps produce energy, transformed through transamination converted back to pyruvate.
• The pentose phosphate pathway, occurs in the cytoplasm generates no ATP and the principal products are NADPH and ribose-5-phosphate and the non-oxidative phase involves the isomerization.
• Three useful intermediates in other pathways: ribose-5-phosphate, fructose-6-phosphate, and glyceraldehyde-3-phosphate.