Carbohydrates Overview

Carbohydrates

• Carbohydrates (saccharides) are the most abundant organic molecules in nature, widely distributed in plants and animals.
• They have vital structural and metabolic roles, providing a significant portion of dietary calories for many organisms, acting as energy storage in the body, and as cell membrane components for intercellular communication.
• The empirical formula for many simpler carbohydrates is (CH₂O)ₙ, where n ≥ 3.

Glucose

• Glucose is the most crucial carbohydrate.
• Dietary carbohydrates are absorbed into the bloodstream as glucose.
• Other sugars are converted to glucose in the liver.
• Glucose is the primary metabolic fuel for mammals.
• It's a precursor for the synthesis of all other carbohydrates in the body, including glycogen for storage, ribose and deoxyribose in nucleic acids (DNA and RNA), galactose for lactose synthesis in milk, and components of glycoproteins and proteoglycans.

Carbohydrate Diseases

• Diseases associated with carbohydrate metabolism include diabetes mellitus, galactosemia, glycogen storage diseases, and lactose intolerance.
• The glycome represents the entire collection of sugars produced by an organism.

Classification of Carbohydrates

• Monosaccharides: Classified by the number of carbon atoms (trioses, tetroses, pentoses, hexoses, heptoses) and whether they have an aldehyde or keto group.
• Disaccharides: Formed from the condensation of two monosaccharides (e.g., lactose, maltose, sucrose, trehalose).
• Oligosaccharides: Formed from the condensation of 3 to 10 monosaccharides.
• Polysaccharides: Formed from the condensation of more than 10 monosaccharides. Examples are glycogen (branched glucose polymer), starches (which can be linear or branched polymers), dextrins, cellulose (plant cell walls), inulin (plant storage carbohydrate), chitin (structural component), and pectin (found in fruits).

Representing the Structure of D-Glucose

• Fischer Projection: Straight-chain form.
• Haworth Projection: α-D-glucose.
• Chair Form: α-D-glucose.

Isomerism of Sugars

• D- and L-isomerism: Isomers with the same chemical formula but different structures.
• Optical Isomerism: (d(+)/l(-)) Molecules that rotate plane-polarized light—either to the right (dextrorotatory—(+)) or left (levorotatory—(-)).
• Pyranose and Furanose Ring Structures: Ring structures of monosaccharides similar to those of pyran (six-membered ring) or furan (five-membered ring).
• Alpha- and Beta-Anomers: Variations in configuration of the -OH and -H groups on the anomeric carbon atoms.
• Epimers: Isomers differing in the configuration around one carbon atom. Examples: glucose-mannose (epimerization at carbon 2), glucose-galactose (epimerization at carbon 4).
• Aldose-Ketose Isomerism: Isomers that differ in the presence of an aldehyde or ketone group.

Reducing Sugars

• Reducing sugars are chemicals that are also reducing substances that are involved in a chemical reaction.
• This reaction enables the basis for a simple chemical test for glucose in poorly managed diabetes—by reduction of an alkaline copper solution. Examples: Glucose, galactose, and mannose.

Glycosides

• Glycosides are comprised of hydroxyl groups, which include different compounds.
• If the hemiacetal portion of this compound is glucose, the outcome is a glucoside; otherwise, it's a galactoside or another variant. An N-glycosidic bond is generated if the second group in the compound is an amine. An example of this type of bond is the bond between adenine and ribose in ATP (adenosine triphosphate) nucleotides.

Glycoconjugates

• Glycoconjugates are important in medicine due to their effects on the heart.
• Cardiac glycosides contain steroids (aglycones). Examples: Digoxin and Ouabain, which inhibit the Na⁺-K⁺-ATPase of cell membranes, and antibiotics like streptomycin.

Deoxy Sugars

• Deoxy sugars are those wherein a hydroxyl group has been replaced by hydrogen. An example is deoxyribose found in DNA.

Amino Sugars

• Amino sugars are components of glycoproteins, gangliosides, and glycosaminoglycans.
• Examples include D-glucosamine, a component of hyaluronic acid, and D-galactosamine (chondrosamine), which is part of chondroitin.
• Several antibiotics, including erythromycin, feature amino sugars.

Disaccharides

• Disaccharides are composed of two monosaccharide units linked by a glycosidic bond. Key examples include maltose, sucrose, lactose, and lactulose.
• Sucrose, maltose, and lactose are examples of significant disaccharides.

Polysaccharides

• Polysaccharides serve various storage and structural functions.
• Examples: starch (amylose and amylopectin), glycogen (animal storage carbohydrate), inulin (plant storage carbohydrate), cellulose (plant cell wall component), chitin (structural component in exoskeletons), and pectin (found in fruits).

Glycemic Index

• Defined as the extent to which a starchy food raises blood glucose levels, compared to an equivalent amount of glucose or a reference food.

Proteoglycans, Glycoproteins and Glycolipids

• Carbohydrate chains (glycans) are attached to proteins (proteoglycans and glycoproteins) or lipids (glycolipids) on the cell's surface playing key roles in functions such as cell recognition, adhesion, migration, immune response, and wound healing.

Glycosaminoglycans (GAGs)

• Glycosaminoglycans are complex carbohydrates that contain amino sugars and uronic acids and are frequently linked to proteins, forming proteoglycans.
• GAGs help hydrate tissues and create space to reduce pressure and support the structure and function of tissues.
• Hyaluronic acid, chondroitin sulfate, keratan sulfate, heparin, and dermatan sulfate are some examples of GAGs.

Glycoproteins

• Proteins (glycoproteins, also known as mucoproteins) have branched or unbranched oligosaccharides covalently bonded to them.
• Glycosylation (sugars attaching to proteins) is a common post-translational modification crucial for protein function.
• Glycoproteins are found in cell membranes, and many proteins are glycosylated—examples include immunoglobulins (antibodies), hormones such as follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH).

Sialic Acids

• Sialic acids are derivatives of neuraminic acid, a nine-carbon sugar. They are involved in glycoproteins and gangliosides.