Introduction to Carbohydrates

• Carbohydrates, also known as saccharides, are a class of naturally occurring compounds and derivatives formed from them.
• They contain carbon, hydrogen, and oxygen, with hydrogen and oxygen typically in a 2:1 ratio
• Carbohydrates serve as a primary source of energy for living organisms and also play crucial structural roles in cells and tissues.
• Carbohydrates include monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

Carbohydrate Structure

• Monosaccharides are the simplest carbohydrates, often called simple sugars.
• They cannot be hydrolyzed into smaller carbohydrates.
• General formula for monosaccharides is (CH2O)n, where n is three or greater.
• Monosaccharides are classified based on the number of carbon atoms they contain: trioses (3 carbons), tetroses (4 carbons), pentoses (5 carbons), hexoses (6 carbons), and so on.
• Monosaccharides are either aldoses (containing an aldehyde group) or ketoses (containing a ketone group), depending on the position of the carbonyl group.
• Examples of aldoses include glucose, galactose, and ribose
• Examples of ketoses include fructose and ribulose.
• Monosaccharides can exist in either a linear or cyclic form.
• In aqueous solution, monosaccharides with five or more carbon atoms predominantly exist in cyclic forms due to intramolecular hemiacetal or hemiketal formation.
• The cyclic forms of monosaccharides are either furanoses (five-membered rings) or pyranoses (six-membered rings).
• During cyclization, a new chiral center is created at the carbonyl carbon, resulting in two possible stereoisomers, α and β anomers.
• The α anomer has the hydroxyl group on the anomeric carbon on the opposite side of the ring from the CH2OH group attached to the chiral center that determines the D or L configuration.
• The β anomer has the hydroxyl group on the same side.
• Disaccharides are carbohydrates composed of two monosaccharides linked together by a glycosidic bond.
• Common disaccharides include sucrose (glucose + fructose), lactose (galactose + glucose), and maltose (glucose + glucose).
• Oligosaccharides contain a small number (3-10) of monosaccharide units linked together.
• Polysaccharides are large polymers consisting of many monosaccharide units linked by glycosidic bonds.
• Polysaccharides can be homopolysaccharides (composed of the same monosaccharide) or heteropolysaccharides (composed of different monosaccharides).
• Examples of homopolysaccharides include starch, glycogen, and cellulose, all made up of glucose monomers but differ in their linkages and branching patterns.
• Starch is the main storage polysaccharide in plants, consisting of amylose (linear α-1,4-linked glucose) and amylopectin (branched α-1,4- and α-1,6-linked glucose).
• Glycogen is the main storage polysaccharide in animals, similar to amylopectin but with more frequent branching.
• Cellulose is a structural polysaccharide in plant cell walls, consisting of β-1,4-linked glucose units.

Glycolysis

• Glycolysis is a metabolic pathway that converts glucose into pyruvate.
• Glycolysis occurs in the cytoplasm of cells and does not require oxygen (anaerobic).
• The main function of glycolysis is to produce energy in the form of ATP (adenosine triphosphate) and NADH (nicotinamide adenine dinucleotide).
• Glycolysis involves a sequence of ten enzymatic reactions, divided into two phases: the energy investment phase and the energy payoff phase.
• In the energy investment phase, two ATP molecules are consumed to phosphorylate glucose, resulting in fructose-1,6-bisphosphate.
• In the energy payoff phase, fructose-1,6-bisphosphate is cleaved into two three-carbon molecules, which are then converted into pyruvate, producing four ATP molecules and two NADH molecules.
• The net yield of glycolysis is two ATP molecules, two NADH molecules, and two pyruvate molecules per glucose molecule.
• Key regulatory enzymes in glycolysis include hexokinase, phosphofructokinase-1 (PFK-1), and pyruvate kinase.
• Hexokinase catalyzes the phosphorylation of glucose to glucose-6-phosphate, trapping glucose inside the cell.
• PFK-1 catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate, a committed step in glycolysis.
• Pyruvate kinase catalyzes the transfer of a phosphate group from phosphoenolpyruvate to ADP, forming pyruvate and ATP.
• The pyruvate produced during glycolysis can be further metabolized in several ways, depending on the availability of oxygen.
• In the presence of oxygen (aerobic conditions), pyruvate is converted to acetyl-CoA, which enters the citric acid cycle for further ATP production.
• In the absence of oxygen (anaerobic conditions), pyruvate is converted to lactate (in animals) or ethanol (in yeast) through fermentation.

Glycosides

• Glycosides are compounds formed when a carbohydrate molecule (sugar) is bonded to another molecule (aglycone) through a glycosidic bond.
• The sugar component can be a monosaccharide or an oligosaccharide.
• The aglycone can be a variety of chemical compounds, such as alcohols, phenols, steroids, or nitrogenous bases.
• The glycosidic bond is formed between the anomeric carbon of the sugar and a hydroxyl group of the aglycone (O-glycoside), or another atom such as nitrogen (N-glycoside), sulfur (S-glycoside), or carbon (C-glycoside).
• O-glycosides are the most common type of glycosides.
• Glycosides are found widely in nature, particularly in plants, where they serve various functions such as storage, transport, protection, and structural support.
• Many pharmaceutically important compounds are glycosides, including cardiac glycosides (e.g., digoxin) and saponins.
• Glycosides can be hydrolyzed by acids or enzymes (glycosidases) to release the sugar and aglycone components.
• Glycosidases are enzymes that catalyze the hydrolysis of glycosidic bonds.
• Glycosylation is the process of adding a carbohydrate to another molecule, such as a protein or lipid.
• Glycosylation is an important post-translational modification of proteins, affecting their folding, stability, localization, and function.
• N-linked glycosylation occurs at asparagine residues, while O-linked glycosylation occurs at serine or threonine residues.

Acetylation

• Acetylation is a chemical reaction in which an acetyl group (CH3CO) is introduced into a molecule.
• Acetylation is commonly used to modify proteins, particularly histones, as well as other biomolecules such as carbohydrates and lipids.
• Acetylation of proteins, especially histones, is a crucial regulatory mechanism in gene expression.
• Histone acetylation generally leads to a more open chromatin structure, promoting gene transcription.
• Histone acetyltransferases (HATs) are enzymes that catalyze the transfer of an acetyl group from acetyl-CoA to histone proteins.
• Histone deacetylases (HDACs) are enzymes that remove acetyl groups from histone proteins, leading to chromatin condensation and gene repression.
• Acetylation can also affect the stability, localization, and activity of non-histone proteins.
• Acetylation of carbohydrates involves the introduction of acetyl groups to hydroxyl groups on the sugar molecule.
• Acetylated sugars are found in various biological contexts and can influence the properties and interactions of carbohydrates.
• Acetylation of lipids can modulate membrane properties and signaling pathways.
• Acetylation is involved in various cellular processes, including DNA replication, DNA repair, cell cycle progression, and apoptosis.