Which one of the following enzyme brings about hydrolysis of lactose to glucose and galactose?

Key Points

• Lactose is used as an energy source in E. coli in absence of glucose.
• Lactose can be hydrolyzed into its monomeric subunits glucose and galactose with the help of β-galactosidase enzyme.
• This enzyme is produced in E. coli by a polycistronic gene that is regulated by the lac operon.

Explanation:

The lac operon consists of the following parts:

• Structural Gene - It consists of 3 genes:
  • z - codes for β-galactosidase
  • y - codes for permease
  • a - codes for transacetylase
• Operator -
  • It is present adjacent to the structural gene.
  • It is the site for the binding of repressor protein.
• Regulator -
  • It comprises of the i-gene, which codes for the repressor protein.
  • The repressor protein is synthesized all the time constitutively.
• Promoter -
  • It is the transcription initiation site where RNA polymerase binds.
• Inducer -
  • It is the molecule that determines whether the repressor will bind to the operator or not.
  • Hence, it regulates the operon.
  • Example - Lactose in lac operon.

Additional Information

Lac Operon Regulation - 

• In the absence of lactose, the repressor protein binds at the operator site and prevents RNA polymerase to transcribe the structural genes.
• But in the presence of lactose, transcription of the genes take place.
• Inducer binds to the repressor and inactivates it by forming the repressor inducer complex.
• This complex prevents the repressor from binding with the operator.
• Thus, the RNA polymerase slides along the DNA without any hindrance and thus, the genes get expressed.