Comprehensive Biochemistry Notes on Carbohydrates

Definition and Chemical Nature of Carbohydrates

• Abundance and Elemental Composition:

  • Carbohydrates represent the most abundant organic molecules found in nature.

  • They are composed primarily of three chemical elements: carbon ($C$), hydrogen ($H$), and oxygen ($O$).

• Etymology and Empirical Formula:

  • The name carbohydrate literally signifies 'hydrates of carbon'.

  • Certain carbohydrates satisfy the empirical formula $(C \times H_2O)_n$ where $n \le 3$, giving the appearance of carbon hydrates.

  • Exceptions and Non-Carbohydrate Hydrates:

  • Several non-carbohydrate chemical compounds share the empirical ratio of carbon hydrates but are not carbohydrates:

    • Acetic acid ($C_2H_4O_2$)

    • Lactic acid ($C_3H_6O_3$)

  • Conversely, multiple genuine carbohydrates do not conform to the general empirical formula $(C \times H_2O)_n$:

    • Rhamnohexose ($C_6H_{12}O_5$)

    • Deoxyribose ($C_5H_{10}O_4$)

  • Consequently, carbohydrates cannot be broadly defined strictly as hydrates of carbon.

• Biochemical Definition:

  • Carbohydrates are defined as polyhydroxyaldehydes or polyhydroxyketones, or complex organic compounds that yield these substances upon hydrolysis.

  • Carbohydrate units are held together in larger molecules via glycosidic bonds.

• Characteristics of Sugars:

  • The term 'sugar' is specifically applied to carbohydrates that are readily soluble in water and possess a sweet taste.

Biological Functions of Carbohydrates

• Dietary Energy Source:

  • Carbohydrates serve as the primary and most abundant dietary source of energy for all living organisms, supplying $4\,\text{Cal/g}$.

• Metabolic Precursors:

  • They act as foundational precursors for the biosynthesis of various other organic compounds, including lipids/fats and amino acids.

• Cellular Membrane and Functional Roles:

  • In the forms of glycoproteins and glycolipids, carbohydrates are essential structural constituents of cell membranes.

  • They actively participate in key cellular functions, including cell growth, intercellular adhesion, and fertilization processes.

• Structural Components:

  • Carbohydrates function as major structural framework constituents in diverse organisms:

  • Cellulose fiber providing structural rigidity in plants.

  • Exoskeletons of insects and arthropods.

  • Cell walls of various microorganisms.

• Energy Storage:

  • Carbohydrates function as the primary short-term storage form of energy (notably glycogen in animals) to rapidly satisfy immediate metabolic energy demands.

Classification of Carbohydrates

• General Terminology:

  • Carbohydrates are frequently designated as saccharides, derived from the Greek word sakcharon, meaning sugar.

  • Carbohydrates are categorized into three major groups based on the number of constituent sugar (monosaccharide) units: monosaccharides, oligosaccharides, and polysaccharides.

• General Sugar Characteristics:

  • Both monosaccharides and oligosaccharides are characterized by a sweet taste, crystalline structure, and high solubility in water, and are broadly known as sugars.

Monosaccharides

• Definition and Properties:

  • Monosaccharides (derived from Greek mono, meaning one) are the simplest carbohydrate units, commonly called simple sugars.

  • They possess the general chemical formula $C_n(H_2O)_n$.

  • Monosaccharides cannot be hydrolyzed further into simpler carbohydrate molecules.

• Classification Criteria:

  • Monosaccharides are classified based on two structural characteristics: the type of functional group and the total number of carbon atoms.

  • Classification by Functional Group:

  • Aldoses: Monosaccharides containing an aldehyde functional group ($\text{-C(=O)H}$). Examples include glyceraldehyde and glucose.

  • Ketoses: Monosaccharides containing a keto functional group ($\text{-C(=O)-}$). Examples include dihydroxyacetone and fructose.

  • Classification by Carbon Atom Count:

  • Trioses ($3\text{C}$)

  • Tetroses ($4\text{C}$)

  • Pentoses ($5\text{C}$)

  • Hexoses ($6\text{C}$)

  • Heptoses ($7\text{C}$)

  • Naming combines the functional group and carbon count. For example, glucose is classified as an aldohexose, whereas fructose is classified as a ketohexose.

• Systematic Classification of Monosaccharides:

  • Trioses ($C_3H_6O_3$):

  • Aldose: Glyceraldehyde

  • Ketose: Dihydroxyacetone

  • Tetroses ($C_4H_8O_4$):

  • Aldose: Erythrose

  • Ketose: Erythrulose

  • Pentoses ($C_5H_{10}O_5$):

  • Aldose: Ribose

  • Ketose: Ribulose

  • Hexoses ($C_6H_{12}O_6$):

  • Aldose: Glucose

  • Ketose: Fructose

  • Heptoses ($C_7H_{14}O_7$):

  • Aldose: Glucoheptose

  • Ketose: Sedoheptulose

Oligosaccharides and Polysaccharides

• Oligosaccharides:

  • Oligosaccharides yield a small number of monosaccharide units upon hydrolysis.

  • Depending on the precise number of monomeric units liberated during hydrolysis, they are subdivided into disaccharides, trisaccharides, tetrasaccharides, and so on.

• Polysaccharides:

  • Polysaccharides (derived from Greek poly, meaning many) are high molecular weight polymers composed of repeating monosaccharide units, with molecular weights reaching up to $10^6$ (one million).

  • Unlike simple sugars, polysaccharides are typically tasteless (non-sugars) and form colloidal dispersions when mixed with water.

  • Polysaccharides are divided into two main categories:

  • Homopolysaccharides: Composed of a single repeating type of monosaccharide unit.

  • Heteropolysaccharides: Composed of two or more different types of monosaccharide units.

Monosaccharides: Structural Aspects and Stereoisomerism

• Stereoisomerism:

  • Stereoisomerism represents a key structural feature of monosaccharide chemistry.

  • Stereoisomers are defined as chemical compounds that share identical structural formulae but differ strictly in their spatial three-dimensional configuration.

• Asymmetric Carbon Atoms:

  • A carbon atom is defined as asymmetric (or chiral) when it is covalently bonded to four completely different atoms or functional groups.

  • The total number of asymmetric carbon atoms, denoted as $n$, directly determines the maximum theoretical number of possible stereoisomers for a given molecule according to the formula:     $\text{Total Isomers} = 2^n$

  • Example in Glucose:

  • Glucose contains $4$ asymmetric carbon atoms ($n = 4$).

  • Applying the formula yields:       $2^4 = 16\,\text{possible stereoisomers}$

• Reference Carbohydrate - Glyceraldehyde:

  • Glyceraldehyde, an aldotriose, represents the simplest monosaccharide containing a single asymmetric carbon atom ($n = 1$).

  • Because of its simple structure, glyceraldehyde serves as the standard reference carbohydrate for establishing stereochemical configurations.

• Structural Components of Glucose:

  • The open-chain Fischer structure of glucose consists of a six-carbon backbone with functional hydroxyl groups ($\text{-OH}$) and hydrogen atoms ($\text{-H}$) arranged across carbon centers (containing repeating $\text{HCOH}$ structural units terminated by a hydroxymethyl group, $\text{HOCH}_2$).