Lecture 1 - Biomolecules PDF

# BIOMOLECULES - Biomolecules are the most essential organic molecules, and the building blocks of all the living organisms. - They range from small molecules such as primary and secondary metabolites, vitamins and hormones to large macromolecules like proteins, nucleic acids, carbohydrates, lipid...

# BIOMOLECULES - Biomolecules are the most essential organic molecules, and the building blocks of all the living organisms. - They range from small molecules such as primary and secondary metabolites, vitamins and hormones to large macromolecules like proteins, nucleic acids, carbohydrates, lipids (Figure1). - [Image Description: a diagram showing a circle labelled "Lipids", a circle labelled "Nucleic Acids", a chain labelled "Carbohydrates" and a sphere labelled "Proteins"] ## Carbohydrates - Carbohydrates (saccharides) are the most abundant organic molecules in nature. - They have a wide range of functions, including: - Providing a significant fraction of the dietary calories for most organisms. - Acting as a storage form of energy in the body. - Serving as cell membrane components that mediate some forms of intercellular communication. - Serve as a structural component of many organisms including the cell walls of bacteria, and the fibrous cellulose of plants. - The general formula for many of the simpler carbohydrates is $(CH_2O)_n$, where $n \geq 3$, hence the name "hydrate of carbon." ## Classification of Carbohydrates - Carbohydrates are classified according to number of sugar units as: - Monosaccharides: 1 sugar unit. - Disaccharides: 2 sugar units. - Oligosaccharides: 3-9 sugar units. - Polysaccharides: 10 or more sugar units. ## Monosaccharides - Monosaccharides (simple sugars) can be classified according to the number of carbon atoms they contain. - Examples of some monosaccharides commonly found in humans are: | Generic names | Examples | |---|---| | 3 Carbons: trioses | Glyceraldehyde | | 4 Carbons: tetroses | Erythrose | | 5 Carbons: pentoses | Ribose | | 6 Carbons: hexoses | Glucose | | 7 Carbons: heptoses | Sedoheptulose | | 9 Carbons: nonoses | Neuraminic acid | - They can also be classified by the type of carbonyl group they contain. - Carbohydrates with an aldehyde as their carbonyl group are called "aldoses” (glucose, galactose, mannose), whereas those with a keto as their carbonyl group are called “ketoses” (fructose). ## Isomers - Compounds that have the same chemical formula but have different structures are called isomers. - For example, fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula, $C_6H_{12}O_6$. ## Epimers - Carbohydrate isomers that differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon) are defined as epimers of each other. - Glucose and galactose are C-4 epimers because their structures differ only in the position of the –OH group at carbon 4. - Glucose and mannose are C-2 epimers. ## Enantiomers - A special type of isomerism is found in the pairs of structures that are mirror images of each other. - These mirror images are called enantiomers, and the two members of the pair are designated as a D- and an L-sugar. - The vast majority of the sugars in humans are D-sugars. - In the D isomeric form, the OH group on the a symmetric carbon (a carbon linked to four different atoms or groups) farthest from the carbonyl carbon is on the right, whereas in the L-isomer, it is on the left. ## Cyclization of Monosaccharides - Less than 1% of each of the monosaccharides with five or more carbons exists in the open chain (a cyclic) Fischer projection formula in solution. - Rather, they are predominantly found in a ring (cyclic) Haworth projection formula. - In which the aldehyde (or keto) group has reacted with an alcohol group on the same sugar, making the carbonyl carbon (C1 for an aldose, C2 for a ketose) a symmetric. - This a symmetric carbon is referred to as the anomeric carbon. ## Anomers - Creation of an anomeric carbon, generates a new pair of isomers, the a and β configurations of the sugar (a-D-glucopyranose and β-D-glucopyranose), that are anomers of each other. - The cyclic a and ẞ anomers of a sugar in solution spontaneously (but slowly) form an equilibrium mixture a process known as mutarotation. - The a and ẞ forms are not mirror images, and they are referred to as diastereomers. ## Reducing Sugars - If the hydroxyl group on the anomeric carbon of a cyclized sugar is not linked to another compound by a glycosidic bond, the ring can open. - The sugar can act as a reducing agent and is termed a reducing sugar. - Such sugars can react with chromogenic a gents (Benedict reagent) causing the reagent to be reduced and colored, with the aldehyde group of the a cyclic sugar becoming oxidized. - All monosaccharides but not all disaccharides, are reducing sugars. - Glucose can have its terminal hydroxyl group oxidized to a carboxyl group, forming gluconic acid, or its anomeric carbon oxidized forming glucuronic acid, or both oxidized forming glucaric acid. ## Disaccharides - A disaccharide (also called a double sugar) is the sugar formed when two monosaccharides (simple sugars) are joined by glycosidic linkage. - The oxide linkage is formed after the loss of the water molecule and then the two monosaccharides are formed by that linkage. - Like monosaccharides, disaccharides are soluble in water. - Three common examples are sucrose, lactose, and maltose. ## Sucrose (table sugar) - The most common disaccharide is sucrose which gives D- glucose and D- fructose on hydrolysis. - Both the monosaccharides i.e. glucose and fructose are connected through the glycosidic linkage between alpha glucose and second carbon beta fructose. - Sucrose is a non-reducing sugar as both the reducing groups of glucose and fructose are involved in the glycosidic bond formation. ## Maltose (Malt sugar) - Maltose is also one of the disaccharides which have two a-D-glucose units which are connected by the first carbon of the glucose and linked to the fourth carbon of another glucose unit. - In the solution, a free aldehyde can be produced at the first carbon of the second glucose of the solution and it is a reducing sugar as it shows reducing properties. ## Lactose (Milk sugar) - Commonly it is called milk sugar as this disaccharide is found in milk. - It is made up of β-D-galactose and a-D-glucose. The bond is between the first carbon of galactose and the fourth carbon of glucose. - This is also a reducing sugar. ## Some More Types of Disaccharides ### Trehalose - It is made up of 2 molecules of glucose which are linked differently. - This can be found in fungi, plants, and insects. ### Lactulose - It is formed from galactose and fructose. - It is helpful for the treatment of constipation and liver diseases. ### Cellobiose - It is also made up of two glucose molecules which are also arranged differently. - These can be seen bacteriology which is a form of chemical analysis. ### Chitobiose - It comprises two glucosamine molecules which are linked. - It is seen in some bacteria, exoskeletons of insects and is also found in fish, octopus, and squid.