4: BIOMOLECULES

• Carbon


  The most predominant and versatile element of life. It possesses a unique property to form infinite number of compounds. This is attributed to the ability of carbon to form stable covalent bonds and C—C chains of unlimited length. It is estimated that about 90% of compounds found in living system invariably contain carbon.
• Complex biomolecules


  • Proteins
  • Nucleic acids (DNA and RNA)
  • Polysaccharides
• Monomers


  Amino acids, nucleotides and monosaccharide's serve as the building blocks of complex biomolecules
• Lipids are not biopolymers in a strict sense, but majority of them contain fatty acids.

  
• Macromolecules (proteins, lipids, nucleic acids and polysaccharides)

  • They form supra-molecular assemblies (e.g., membranes) which in turn organize into organelles, cells, tissues, organs and finally the whole organism.

  
• Carbohydrates


  The most abundant organic molecules in nature. They are primarily composed of the elements carbon, hydrogen and oxygen. The name carbohydrate literally means 'hydrates of carbon.' Carbohydrates may be defined as polyhydroxy- aldehydes or ketones or compounds which produce them on hydrolysis.

  
• Sugars


  Carbohydrates soluble in water and sweet to taste.
• Classification of Carbohydrates

  • Monosaccharide's
  • Oligosaccharides
  • Polysaccharides

  
• Monosaccharide's


  The simplest group of carbohydrates and are often referred to as simple sugars. They have the general formula Cn(H2O)n, and they cannot be further hydrolysed.

  
• Oligosaccharides


  Contain 2-10 monosaccharide molecules which are liberated on hydrolysis.

  
• Polysaccharides


  Polymers of monosaccharide units with high molecular weight (up to a million). They are usually tasteless (non-sugars) and form colloids with water.
• Types of Polysaccharides


  • Homopolysaccharides
  • Heteropolysaccharides
• Stereoisomerism


  An important character of monosaccharides. Stereoisomers are the compounds that have the same structural formulae but differ in their spatial configuration.
• Asymmetric carbon


  A carbon is said to be asymmetric when it is attached to four different atoms or groups.
• Glyceraldehyde


  The simplest monosaccharide with one asymmetric carbon atom. It exists as two stereoisomers, and has been chosen as the reference carbohydrate to represent the structure of all other carbohydrates.

  
• 1. and L-isomers


  Mirror images of each other. The special orientation of —H and —OH groups on the carbon atom (C5 for glucose) that is adjacent to the terminal primary alcohol carbon determines whether the sugar is D- or L-isomer.

  
• Optical activity


  A characteristic feature of compounds with asymmetric carbon atom. When a beam of polarized light is passed through a solution of an optical isomer, it will be rotated either to the right or left.
• Glycosides


  Formed when the hemiacetal or hemiketal hydroxyl group (of anomeric carbon) of a carbohydrate reacts with a hydroxyl group of another carbohydrate or a non-carbohydrate (e.g. methyl alcohol, phenol, and glycerol). The bond so formed is known as glycosidic bond and the non- carbohydrate moiety (when present) is referred to as aglycone.

  
• Derivatives of Monosaccharide's


  • Amino sugars
  • Deoxysugars
  • L-Ascorbic acid (vitamin C)

  
• Amino sugars


  When one or more hydroxyl groups of the monosaccharide's are replaced by amino groups.
• Deoxysugars


  Sugars that contain one oxygen less than that present in the parent molecule.
• 1. Ascorbic acid (vitamin C)


  A water- soluble vitamin, the structure of which closely resembles that of a monosaccharide.

  
• Types of Disaccharides


  • Reducing disaccharides with free aldehyde or keto group
  • Non-reducing disaccharides with no free aldehyde or keto group
• Homopolysaccharides


  On hydrolysis yield only a single type of monosaccharide. They are named based on the nature of the monosaccharide unit.

  
• Heteropolysaccharides


  On hydrolysis yield a mixture of a few monosaccharide's or their derivatives.

  
• Starch


  • The carbohydrate reserve of plants which is the most important dietary source for higher animals, including man. It is a homopolymer composed of D-glucose units held by α glycosidic bonds. It consists of two polysaccharide components-water soluble amylose (15-20%) and a water insoluble amylopectin (80-85%).
• Glycogen


  • The carbohydrate reserve in animals, hence often referred to as animal starch. It is present in high concentration in liver, followed by muscle, brain etc. The structure of glycogen is similar to that of amylopectin with more number of branches.

  
• Cellulose


  • Occurs exclusively in plants and it is the most abundant organic substance in plant kingdom. It is a predominant constituent of plant cell wall. Cellulose is composed of β-D-glucose units linked by β (1 → 4) glycosidic bonds. Cellulose cannot be digested by mammals—including man—due to lack of the enzyme that cleaves β-glycosidic bonds.
• Mucopolysaccharides


  Heteroglycans made up of repeating units of sugar derivatives, namely amino sugars and uronic acids. They are more commonly known as glycosaminoglycan's (GAG). Acetylated amino groups, besides sulfate and carboxyl groups are generally present in GAG structure.

  
• Lipids


  Of great importance to the body as the chief concentrated storage form of energy.
• Polysaccharides


  More commonly known as glycosaminoglycans (GAG)

  
• GAG structure


  • Acetylated amino groups, besides sulfate and carboxyl groups are generally present

  
• Mucoproteins


  Combination of mucopolysaccharides and proteins
• Mucoproteins may contain up to 95% carbohydrate and 5% protein
• Mucopolysaccharides are essential components of tissue structure
• Extracellular spaces of tissue


  • Consist of collagen and elastin fibers embedded in a matrix or ground substance
  • Ground substance is predominantly composed of GAG
• Important mucopolysaccharides


  • Hyaluronic acid
  • Chondroitin 4-sulfate
  • Heparin
  • Dermatan sulfate
  • Keratan sulfate

  
• Lipids


  Organic substances relatively insoluble in water, soluble in organic solvents (alcohol, ether etc.), actually or potentially related to fatty acids and utilized by the living cells

  
• Unlike the polysaccharides, proteins and nucleic acids, lipids are not polymers
• Classification of lipids


  • Simple lipids
  • Complex (or compound) lipids
  • Derived lipids
  • Miscellaneous lipids
  • Neutral lipids