Slide 1 : WELCOME TO PLANETUTOR
Slide 2 : BIOMOLECULES CELLULAR
MACROMOLECULES
CONTENTS : CONTENTS INTRODUCTION
POLYSACCARIDES
PROTEINS
NUCLEIC ACIDS
DNA
RNA
ENZYMES AND INORGANIC CATALYSTS
CLASSIFICATION OF ENZYMES
SUMMARY
INTRODUCTION : INTRODUCTION The living cells contain specific macromolecules with a high molecular weight and a poor solubility.
Generally in a cell, macromolecules are represented by polysaccharides, proteins, nucleic acids and enzymes.
These compounds are formed by polymerisation of micromolecules such as sugars, amino acids and nucleotides
POLYSACCHARIDES : POLYSACCHARIDES Polysaccharides are complex carbohydrates formed from monosaccharides. A number of monosaccharide molecules such as those of glucose, become linked by glycosidic bonds with the elimination of a molecule of water for each monosaccharide added.
When a polysaccharide has multiple molecules of the same type, it is described as homopolysaccharide. For example, starch and glycogen are composed of only glucose.
When a polysaccharide molecule is formed by more than one type of monosaccharide molecules, it is described as a heteropolysaccharide. For example, chitin and agar
Slide 6 : Structural Appearance of Polysaccharides
PROTEINS : PROTEINS Proteins, which constitute about 10 to 12% of the cell contents, are macromolecules having a significant role in the metabolism of a cell.
Proteins are polymers of amino acids. Each protein is a linear, unbranched, polypeptide molecule.
The order in which amino acids occur is specific for each polypeptide. The protein molecules are very large and highly complex macromolecules.
Insulin (human) has 53 amino acids arranged in two polypeptide chains of 22 and 31 amino acids. Human serum albumin has 582 amino acids in its polypeptide chain.
Slide 8 : Proteins show four structural levels namely, primary, secondary, tertiary and quaternary.
The linear sequence of amino acids in a polypeptide chain represents the primary structure. The enzyme ribonuclease and the protein myoglobin function only in their primary structure.
If the polypeptide chain is coiled into a spiral or helix to have a three-dimensional structure, then it is called secondary structure. e.g., keratin of skin.
If the helical polypeptide molecule is folded on itself assuming a complex but specific structure such as spherical or rod like, then it is called tertiary structure. e.g., globulins of blood.
Some proteins have two or more polypeptides, each with primary, secondary and tertiary structures then it is called quaternary structure. e.g., Insulin and hemoglobin.
Slide 9 : Levels of Structure in Proteins
NUCLEIC ACIDS : NUCLEIC ACIDS Nucleic acids are giant macromolecules having a complex structure and very high molecular weight. They are polymers of nucleotides and hence, described as polynucleotides.
There are two types of nucleic acids found in a cell, namely deoxy ribose nucleic acid (DNA) and ribose nucleic acid (RNA).
In a nucleic acid molecule, the phosphate component attached to the 5th carbon of one nucleotide forms a linkage with the next nucleotide at the 3rd carbon. This linkage is called phospho-diester linkage and the bond formed is called phospho-diester bond. These 3-5 phospho-diester bonds provide considerable rigidity to the polynucleotide chain.
DNA : DNA With the exception of a few viruses, DNA forms the genetic material in all living organisms. In the prokaryotic cells DNA occurs in the cytoplasm.
In the eukaryotic cells it is confined largely to the nucleus and forms the main component of chromosomes.
A small amount of DNA is found in the cytoplasm in the organelles like mitochondria and plastids. It is called extra- nuclear DNA.
RNA : RNA Ribose Nucleic Acid (RNA)?
RNA occurs mostly in the cytoplasm in the eukaryotic cells. A small amount occurs in the nucleus of the cell, as a constituent of nucleolus.
RNA is a single polynucleotide chain composed of nucleotides of adenine, guanine, cytosine and uracil. Thymine nucleotides are absent.
There are three types of RNA.
Messenger RNA (mRNA)?
It represents about 5 to 10% of the total RNA. It is synthesised from DNA as and when necessary. It carries the genetic information in the form of a specific sequence of nitrogen bases arranged in triplet codons, which are copies from the code in DNA.
Slide 13 : Transfer RNA (tRNA)?
It represents about 10 to 15% of the total RNA in the cell. It has the shortest molecule having only about 80 to 100 nucleotides. The polynucleotide chain is folded on itself to have the shape of a cloverleaf. The molecule has three lateral loops -a DHU loop, a t loop and an anticodon loop. The anticodon loop bears a triplet combination of nitrogen bases, called anticodon. It is complementary to a codon of mRNA.
The tRNA molecule is meant for recognising and carrying particular types of amino acids to the sites of protein synthesis.
Ribosomal RNA (rRNA)?
It represents nearly 80% of the total RNA in the cell. It always occurs bound to basic proteins in ribosomes. It takes part in assembling the amino acids brought by tRNA, into a polypeptide chain, based on the sequence of codons in mRNA.
Enzymes and Inorganic Catalysts : Enzymes and Inorganic Catalysts Enzymes resemble inorganic catalysts in several aspects and differ from them in many other features. These similarities and differences are summarised in the following table.
Slide 15 : Mode of Enzyme Action
There are two different views to explain the mode of enzyme action the lock and key hypothesis and the induced-fit hypothesis.
Lock and Key Hypothesis
The enzyme molecule provides a uniquely structured template on which the substrate molecules can become attached and interact subsequently. This brings about an interaction between the specific active sites in the enzyme molecule and the reactive sites in the substrate molecule.
The enzyme now breaks down the substrate into products. The products initially remain attached to the enzyme for a short while forming an enzyme product complex. The products get released from the enzyme molecule subsequently.
The enzyme is now ready to receive another substrate molecule again. Thus, the same enzyme can be used again and again.
Slide 16 : Lock and Key Hypothesis
Induced Fit Hypothesis : Induced Fit Hypothesis According to this hypothesis, there is an intermediate condition called transition state between the substrate and the products. It is highly unstable.
When the substrate molecules bind to the enzyme molecule, a change is brought about in the active site to precisely fit the transition state (induced fit). This induced fit hold the substrates at the correct angle for the reaction to take place.
The fact that an active site may also have a conformation to fit the product helps in explaining the role of enzymes in catalysing reversible reactions.
Classification of Enzymes : Classification of Enzymes Enzymes are generally classified on the basis of the type of reactions that they catalyse. 6 groups of enzymes can be recognised on this basis.
Summary : Summary The macromolecules found in a cell are large molecules with a complex structure and very high molecular weight. They are represented by polysaccharides, proteins and nucleic acids.
Slide 20 : THANK YOU