Introduction to Carbohydrates

Carbohydrate Introduction

• Carbohydrates can be defined as polyhydroxy-aldehydes or polyhydroxy-ketones or derivatives of polyhydroxy alcohols.
• They are composed of carbon, hydrogen, and oxygen.
• They are also referred to as saccharides.
• Carbohydrates are the main energy source for the body.

Classification of Carbohydrates

• Monosaccharides consist of a single polyhydroxy aldehyde or polyhydroxy ketone unit.
• Disaccharides are composed of two monosaccharide units linked by a glycosidic bond.
• Oligosaccharides have 3-10 monosaccharide units.
• Polysaccharides are polymers of more than 10 monosaccharides, which can be linear (e.g., cellulose) or branched (e.g., glycogen).

Monosaccharides

• Monosaccharides are classified based on the number of carbon atoms and the active group.
• Trioses (3 carbons) have either aldehyde or ketone groups.
• Tetroses (4 carbons) have either aldehyde or ketone groups.
• Pentoses (5 carbons) have either aldehyde or ketone groups.
• Hexoses (6 carbons) have either aldehyde or ketone groups.

Characteristics of Monosaccharides

• Isomers are compounds with the same formula but different arrangements in space.
• Stereoisomers are isomers that are non-superimposable mirror images.
• Aldo-keto isomers differ in functional groups (aldehyde vs. ketone).
• Epimers differ in the position of an OH group around one carbon.
• Enantiomers are mirror-image isomers.
• Anomers differ at the anomeric carbon, the new chiral center formed in the ring structure.

Disaccharides

• Maltose is composed of two glucose molecules linked by an α(1→4) glycosidic bond.
• Isomaltose is composed of two glucose molecules linked by an α(1→6) glycosidic bond.
• Cellobiose is composed of two glucose molecules linked by a β(1→4) glycosidic bond.
• Lactose is composed of galactose and glucose linked by a β(1→4) glycosidic bond.
• Sucrose is composed of glucose and fructose linked by an α,β(1→2) glycosidic bond.
• Lactulose is composed of galactose and fructose linked by a β(1→4) glycosidic bond.

Polysaccharides

• Glycogen, starch, and cellulose are important examples of polysaccharides.
• Glycogen is a branched polymer of glucose that serves as a storage form of glucose in animals.
• Starch is a storage polysaccharide in plants, composed of two components, amylose (linear) and amylopectin (branched).
• Cellulose is a structural polysaccharide in plants, composed of β-(1→4) linked glucose units.
• Hyaluronic acid is a component of connective tissues, composed of alternating N-acetyl glucosamine and glucuronic acid residues.
• Keratan sulfate is a glycoaminoglycan composed of N-acetylgalactosamine-4-sulfate and galactose.
• Heparin and heparin sulfate are anticoagulants found in mast cells, composed of glucosamine and glucuronic acid.

Chemical Properties of Carbohydrates

• Reducing properties: All monosaccharides are reducing sugars, due to the presence of an aldehyde or ketone group.
• Amino sugars: replacing an OH group with an amino group.
• Deoxy sugars: replacing an OH group with a hydrogen.
• Sugar acids: by oxidation.

Sugar alcohols

• Aldehyde/Ketone group in sugar can be reduced to alcohol.
• Examples are; sorbitol, mannitol and ribitol.

Carbohydrates: Optical Activity

• Optical activity of a solution arises from asymmetric carbon atoms.

• All monosaccharides (except dihydroxyacetone) are optically active.

• D- or L- enantiomers rotate plane-polarized light in opposite directions.

• α and β anomers rotate plane-polarized light in the same direction but at different angles.

•  Glycoconjugates are carbohydrates covalently linked to proteins or lipids.

• Proteoglycans are a type of glycoconjugate.

• Glycoproteins include proteins with covalently attached oligosaccharides.

• Glycolipids include membrane lipids with attached oligosaccharides.

Proteins Chemistry

• Proteins are polyamides made up of amino acid monomers connected by peptide bonds.
• Amino acids are the building blocks of proteins.
• Amino acids are classified based on their R-group properties and side chains
• Some amino acids (isoleucine, leucine, valine) are Branched-chain amino acids (BCAAs)
• Some amino acids contain Sulfur
• Proteins have four different structural levels.
• Primary structure: The linear sequence of amino acids in a polypeptide chain..
• Secondary structure: The folding of the polypeptide chain into repeating structural motifs such as α-helices or β-sheets.
• Tertiary structure: The three-dimensional arrangement of the entire polypeptide chain.
• Quaternary structure: The structure formed by the assembly of multiple polypeptide chains.

Protein Denaturation

• Denaturation is the loss of a protein's 3-dimensional structure and biological activity due to factors.
• Causes of Denaturation: Physical factors (e.g., high temperature, high pressure), chemical factors (e.g., strong acids or bases, urea).

Enzymes

• Enzymes are biological catalysts, increasing reaction rates without being consumed.
• Enzyme nomenclature utilizes both common names (e.g., sucrase) and systematic names (e.g., α-D-fructofuranosidase).
• Enzymes are classified into six major classes based on the type of reaction they catalyze.
• Enzyme activity is affected by temperature, pH, substrate concentration, and enzyme concentration.

Lipids

• Lipids are a diverse group of hydrophobic molecules with diverse structures and functions.
• They are classified into simple, complex, and derived lipids.
• Fatty acids are important components of lipids, with common classifications based on their chain length and the presence or absence of double bonds.
• Essential fatty acids are those that must be supplied in the diet because they cannot be synthesized in the body.
• Triglycerides are esters of glycerol and fatty acids and are the major form of energy storage in animals.
• Phospholipids are essential components of cell membranes, with a hydrophilic head and hydrophobic tails.
• Steroids are lipids with a four-ring structure.
• Cholesterol is a crucial steroid and the precursor for other steroid hormones.

Lipoproteins

• Lipoproteins are complexes of lipids and proteins that transport lipids in the blood.
• Four major classes of lipoproteins are chylomicrons, VLDL, LDL, and HDL..
• These lipoproteins differ in their lipid and protein composition, size, and function.

Biological Membrane Proteins

• Membranes are important biological structures that separate cells from their surroundings.
• Membranes are highly selective to control the flow of information.
• Membrane structure is characterized by a phospholipid bilayer and embedded proteins.

Metabolism

• Metabolism is the sum of all biochemical reactions inside living organisms.
• Catabolic reactions break down large molecules into smaller ones (releasing energy).
• Anabolic reactions build up large molecules from smaller ones (requiring energy).
• Amphibolic pathways are involved in both catabolic and anabolic reactions.
• Metabolic pathways are a series of chemical reactions that occur in a cell.
• Metabolic pathways can be regulated by controlling the activity or amount of enzymes.