Biochemistry 1: Carbohydrates Overview

• Biochemistry 1 for second-level PharmD students will cover chemistry of carbohydrates, proteins, and lipids.
• The course will also cover enzymes, porphyrin and heme metabolism, and oxidative phosphorylation.
• The final exam is worth 50 marks, practical exams are worth 10 marks, and mid-term exams are worth 15 marks.
• The course work is worth 25 marks.

Overview of Carbohydrates

• Carbohydrates are organic compounds containing carbon, hydrogen, and oxygen, with the formula Cₙ(H₂O)ₙ.
• They include polyhydroxy aldehydes and ketones and yield these compounds upon hydrolysis.
• Carbohydrates are a major source of energy, providing about 50% of the body's energy.
• Mucopolysaccharides, proteoglycans, and glycoproteins, which are structural carbohydrates, are vital components of cell membranes and connective tissues.
• Phosphorylated pentoses like ribose-5-phosphate and deoxyribose-5-phosphate are involved in the structures of nucleosides, nucleotides, and nucleic acids such as RNA and DNA.
• Cellulose, a non-digestible carbohydrate, adds bulk to stools and promotes intestinal movement, thus preventing constipation.
• Heparin is a natural anticoagulant.

Classification of Carbohydrates

• Carbohydrates are classified into monosaccharides, oligosaccharides, and polysaccharides.
• Monosaccharides are simple sugars that cannot be further simplified by hydrolysis.
• Oligosaccharides consist of a few (2-10) monosaccharide residues linked covalently.
• Polysaccharides are polymers containing more than 10 monosaccharide residues and consist of chains of monosaccharide or disaccharide units.

Monosaccharides

• Monosaccharides are classified based on the number of carbon atoms they contain.
  • Trioses have 3 carbons.
  • Tetroses have 4 carbons.
  • Pentoses have 5 carbons.
  • Hexoses have 6 carbons.
  • Heptoses have 7 carbons.
• Monosaccharides are either aldehydes (aldoses) or ketones (ketoses).
• Trioses examples:
  • Glyceraldehyde
  • Dihydroxyacetone
• Monosaccharides are numbered starting from the aldehyde group (carbon 1), but when a ketone group is present, the numbering starts at carbon 2.

Ring Structure of Sugars

• The simple, open-chain formula of sugars can not fully explain some reactions of glucose; hence, cyclic structures are formed.
• In solution, sugars with aldehyde groups undergo hydration to form alcohols.
• Intramolecular reactions lead to condensation between the -OH group of the aldenol and the -OH group of carbon 4 or 5, forming a hemiacetal structure.
• The carbonyl group becomes an asymmetric carbon atom which leads to either alpha or beta form.
• If the remaining -OH is on the right side, it is an α-sugar; if on the left, it is a β-sugar.
• The 1-5 ring form is called pyranose, resembling pyran.
• The 1-4 ring form is called furanose, resembling furan.

Haworth and Chair Formulas

• Cyclic sugar structures exist in Haworth and chair forms.
• In Haworth formulas, -OH groups on the right in the old structure are written downward, while those on the left are written upward.
• In solution, glucose is mainly (99%) present as glucopyranose and only 1% as glucofuranose.
• 37% of glucopyranose is present as the α-D form, and 63% as the β-D form.

Isomers

• Isomers are compounds with the same chemical formula but different structures.
• The presence of an asymmetric carbon atom allows for different isomers.

Asymmetric Carbon Atom

• This is a carbon atom attached to four different groups.
• All monosaccharides have at least one asymmetric carbon atom, except dihydroxyacetone.

Optical Activity

• The ability of a substance to rotate plane-polarized light either to the right or to the left.
• If a substance rotates plane-polarized light to the right, it is dextrorotatory or d- or (+).
• If a substance rotates plane-polarized light to the left, it is levorotatory or (l) or (-).
• Glucose has 4 asymmetric carbon atoms and is dextrorotatory, known as dextrose.
• Fructose has 3 asymmetric carbon atoms and is levorotatory, known as levulose.
• A racemic mixture contains equal numbers of molecules of two optically active sugars, one dextrorotatory and one levorotatory, showing no optical activity.
• Resolution is the separation of an optically inactive racemic mixture into its optically active substances.

Optical Isomerism

• Substances can exist in more than one form (isomer).
• A substance containing one asymmetric carbon atom can exist in several isomers (= 2ⁿ), where n is the number of asymmetric carbon atoms.
• Glucose, with 4 asymmetric carbon atoms, has 2⁴ = 16 isomers.

Enantiomers

• Sugars that are mirror images of one another are known as enantiomers.
• For sugars with more than one chiral center, the D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group.
• Most naturally occurring sugars are D isomers.

Anomeric Carbon and Anomers

• An anomeric carbon is the asymmetric carbon atom derived from the active carbonyl sugar group.
  • Carbon number 1 in aldoses.
  • Carbon number 2 in ketoses.
• Anomers are isomers resulting from the α or β position of the hydroxyl group attached to the anomeric carbon; for example, α and β glucose are anomers.
• In the α-form, the -OH group attached to the anomeric carbon is on the right side.
• In the β-form, the -OH group is on the left side.

Epimers

• An epimeric carbon is an asymmetric carbon atom in a monosaccharide besides the carbon of aldehyde or ketone group.
  • Carbons 2, 3, and 4 of glucose.
• Epimers are isomers differing in the position of -H and -OH groups around only one epimeric carbon.

Aldose and Ketose

• Monosaccharides can be classified as aldoses (containing an aldehyde group) or ketoses (containing a ketone group).