Biochemistry Chapter 11: Carbohydrates – Structure, Function, and Biochemical Roles

Chapter 11: Carbohydrates

Learning Objectives

• Describe the main roles of carbohydrates in nature.

• Define carbohydrate and monosaccharide.

• Explain how simple carbohydrates are linked to form complex carbohydrates.

• Describe how carbohydrates are linked to proteins.

• Identify the three main classes of glycoproteins and explain their biochemical roles.

• Define lectins and outline their biochemical functions.

Chapter Outline

• Monosaccharides Are the Simplest Carbohydrates

• Monosaccharides Are Linked to Form Complex Carbohydrates

• Carbohydrates Can Be Linked to Proteins to Form Glycoproteins

• Lectins Are Specific Carbohydrate-Binding Proteins

Monosaccharides: The Simplest Carbohydrates

Definition and Structure

Monosaccharides are the most basic units of carbohydrates. They are either aldehydes or ketones that contain two or more hydroxyl (–OH) groups. The smallest monosaccharides have three carbon atoms.

• Aldose: Monosaccharide with an aldehyde group (e.g., D-glyceraldehyde).

• Ketose: Monosaccharide with a ketone group (e.g., dihydroxyacetone).

• General formulas:

  • Aldehyde: $\mathrm{R{-}CHO}$

  • Ketone: $\mathrm{R{-}CO{-}R'}$

Key Point: Monosaccharides exist in many isomeric forms, which greatly increases their diversity and biological function.

Common Monosaccharides

Monosaccharides vary in the number of carbon atoms and the arrangement of their functional groups. Some important examples include:

• D-Ribose and D-Deoxyribose: Components of RNA and DNA, respectively.

• D-Glucose: The primary energy source for most organisms.

• D-Mannose and D-Galactose: Important in glycoprotein and glycolipid synthesis.

• D-Fructose: Found in fruits and honey; a component of sucrose.

Isomeric Forms of Carbohydrates

Carbohydrates can exist in several isomeric forms, which are classified as follows:

• Constitutional Isomers: Same molecular formula, different connectivity (e.g., dihydroxyacetone vs. D-glyceraldehyde).

• Stereoisomers: Same connectivity, different spatial arrangement.

  • Enantiomers: Non-superimposable mirror images (e.g., D-glyceraldehyde vs. L-glyceraldehyde).

  • Epimers: Differ at one of several asymmetric carbon atoms (e.g., D-glucose vs. D-mannose).

  • Diastreoisomers: Isomers that are not mirror images (e.g., D-altrose vs. D-glucose).

  • Anomers: Isomers that differ at a new asymmetric carbon atom formed on ring closure (e.g., $\alpha$-D-glucose vs. $\beta$-D-glucose).

Example: The difference between $\alpha$ and $\beta$ anomers of glucose is the position of the hydroxyl group at the anomeric carbon (C-1).

Comparison Table: Types of Isomers in Carbohydrates

Type of Isomer	Definition	Example
Constitutional Isomer	Same formula, different connectivity	Dihydroxyacetone vs. D-glyceraldehyde
Enantiomer	Mirror images	D-glyceraldehyde vs. L-glyceraldehyde
Epimer	Differs at one chiral center	D-glucose vs. D-mannose
Diastreoisomer	Not mirror images	D-altrose vs. D-glucose
Anomer	Differs at anomeric carbon	$\alpha$-D-glucose vs. $\beta$-D-glucose

Key Properties and Functions of Carbohydrates

• Energy Storage: Glucose and its polymers (glycogen, starch) are major energy reserves.

• Structural Roles: Cellulose in plants and chitin in arthropods provide structural integrity.

• Cell Recognition: Oligosaccharides on cell surfaces mediate cell-cell interactions.

• Component of Nucleic Acids: Ribose and deoxyribose are part of RNA and DNA.

Definitions

• Carbohydrate: Organic molecule consisting of carbon, hydrogen, and oxygen, typically with the formula $(CH_2O)_n$.

• Monosaccharide: The simplest carbohydrate, consisting of a single sugar unit.

Example: D-Glucose

• Structure: $C_6H_{12}O_6$

• Function: Main energy source for cells; precursor for many biosynthetic pathways.

Additional info: Later sections of the chapter (not shown in these slides) cover linkage of monosaccharides to form oligo- and polysaccharides, glycoprotein formation, and lectin function.