Question

1) Define and compare the four levels of protein organization. How does the sequence of nucleotides of the gene relate to the levels of protein organization?

2) Explain transcriptional control of gene expression using the expression of the lac operon in Escherichia coli

Answer 1

Four level of protein organisations are :

1. Primary Structure:

The linear sequence of amino acids forming the backbone of protein.

Each protein has a unique sequence of amino acids which is determined by the genes contained in DNA. The primary structure of a protein is largely responsible for its function.In case of primary structure of protein amino acids are held together by a protein covalent peptide bond or linkage.

2. Secondary structure :

The spatial arrangement of protein by twisting of the polypeptide chain.

the conformation of polypeptide chain by twisting of folding is referred to as secondary structure. The amino acids are located close to each other in their sequence. Two types of secondary structures Alpha helix and beta sheet are mainly identified.

Alpha helix:

Alpha helix is the most common spiral structure of a protein. It has a rigid arrangement of polypeptide chain.

Beta pleated sheet:

This is the second type of structure proposed by Pauling and Corey.beta pleated sheet are composed of two or more segments of fully extended peptide chains.

3. Tertiary structure :

The the three-dimensional structure of a functional protein.

it is a compact structure with hydrophobic side chains held interior while the hydrophobic groups are on the surface of the protein molecule. This type of arrangement ensure stability of the molecule.

4. Quaternary Structure :

Some of the proteins are composed of two or more polypeptide chains referred to as subunits. The spatial arrangement of the subunits is known as quaternary structure.

• Linear nucleotide sequence in gene is directly translated into the linear primary structure of protein.So primary structure completely corresponds to the nucleotide sequence.

The folding of proteins causes the secondary and the tertiary structures which are dictated by the activity of the H bonds covalent bonds etc. between primary amino acid residues that in turn depends on neucleotide sequence.So there is indirect relationship.

2. Lac operon :

The lac lac operon consists of a regulatory gene (I), operator gene (O) and three structural gene (Z , Y, A).besides these genes there is a promoter site (P) next to the operator gene where the enzyme RNA polymerase binds. The structure genes Z ,Y and A respectively code for the enzymes beta- galactosidase , galactose permease and galactose acetylase. Beta galactosidase hydrolyses lactose to galactose and glucose while permease is responsible for the transport of lactose into the cell.

Repression of lac operon:

The regulatory gene (I) is constitutive. It is expressed at a constant rate leading to the synthesis of lac repressor. Lac repressor is a tetrameric regulatory protein which specifically binds to the operator gene (O). This prevents the binding of the enzyme RNA polymerase to the promoter site (P) thereby blocking the transcription of structural genes ( Z, Y and A). Thid is what happens in the absence of lactose in E. coli. gadi pressure molecule acts as a negative regulator of the gene expression.

Depression of lac operon:

In the presence of lactose ( inducer) in the medium , a small amouny of it can enter the E.coli cells. The repressor molecules have a high affinity for lactose. The lactose molecules bind and induce a conformational change in the repressor. the result is that the depression gets in activated and therefore cannot bind to the operator gene (O). The RNA polymerase attaches to the DNA at the promoter site and transcription precedes leading to the formation of poly cistronic mRNA ( for genes. Z ,Y and A) and finally the 3 enzymes. Thus lactose induces the synthesis of the three enzymes beta galactosidase, galactoside permease and galactoside acetylase. Lactose acts by in activating the repressor molecules in the process is known as depression of Lac operon.