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.

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.

RNA serves the following functions:

• mRNA has a significant role in genetic code.

• tRNA is responsible for transferring amino acids to the site of protein synthesis (ribosomes).

• rRNA assembles the amino acids into a polypeptide chain. It also serves as a primer for replication of DNA.

• RNA serves as the genetic material in some plant viruses.

Enzymes

A large number of chemical reactions will be taking place in a cell. The rate and efficiency with which these reactions go on in a cell, depend on a group of macromolecules called enzymes. Enzymes are thus, biological catalysts.

Enzymes are synthesised by living cells. The DNA of a cell has the necessary information for the synthesis of all the enzymes required by the cell. Enzymes not only catalyse reactions inside the cell, but also retain this capacity even after they are extracted from the cell.

How do Enzymes speed up Biochemical Reactions?

Any chemical reaction requires a certain amount of energy to get initiated. This is called activation energy. Most of the molecules in a substrate have average kinetic energy, some have high energy and some have low energy. Under normal conditions of temperature only molecules with higher energy react to form the product and hence the rate of reaction is considerably less. The rate of reaction may increase if the temperature increases since heat increases the kinetic energy of molecules. The other method, by which the rate of a reaction can be increased, is to add an enzyme. It lowers the activation energy and allows a large number of molecules to undergo reaction at the same time.

An enzyme catalysed reaction is known to be 10million times faster than a non-catalysed reaction. A single molecule of carbonic anhydrase, an enzyme produced in RBC, hydrates 36 million molecules of carbon dioxide in one minute!

fig. 14.11- Energy Diagram Showing a Catalysed and Uncatalysed Chemical Reaction

Properties of Enzymes

Enzymes in general exhibit the following properties

• Enzymes are basically proteins. They are globular proteins with a very huge molecular weight. For example, the molecular weight of pyruvic dehydrogenase is 4,600,000!

• Enzymes can only accelerate a chemical reaction.

• Enzymes are not used up in the reaction that they catalyse.

• Enzyme controlled reactions are reversible. However, reversion can be checked by removing the products as soon as they are formed.

• Most enzymes have a high turnover number which refers to the number of molecules of the substrate acted upon by a single molecule of the enzyme, per minute.

• Enzymes are highly specific about the substrate on which they act. For example the enzyme maltose can breakdown only maltose and not any other disaccharide or carbohydrate.

• Enzymes are highly sensitive to temperature (thermolabile). The most ideal temperature for enzyme activity is in the range of 25^o to 40^oC. Enzyme activity decreases below and above this range of temperature.

• Each enzyme requires a specific pH for its activity. Enzyme activity decrease below and above this optimum pH.

• Enzymes generally work in groups in a cell. The product one enzyme controlled reaction will serve as the substrate for the next.

• Enzymes get easily destroyed by certain chemical substances such as cyanides and phosphides.