Problem 20 - Biochemistry: Concepts and Connections

Question

In studying the mechanism of a particular enzyme, for which the cloned gene is available, you wish to change a putative active site histidine residue to a proline by site-directed mutagenesis. The His codon to be changed is \(5^{\prime}\)-CAC. You wish to change it to \(5^{\prime}-\mathrm{CCC}\), one of four Pro codons. The nucleotide sequence surrounding the His codon is \(5^{\prime}\)-CTGGAATCTCACTTTATCTGG-3'. Write the nucleotide sequence of an oligonucleotide (a 21-mer) that could force the conversion of the CAC codon to \(\mathrm{CCC}\) in a site-directed mutagenesis operation.

Step-by-Step Solution

Answer

5'-CTGGAATCTCCCTTTATCTGG-3'

Step 1: Identify the Target Codon

The problem requires changing a histidine (His) codon, which is specified as 5'-CAC. We need to identify where this codon appears in the sequence and replace it.

Step 2: Locate the Codon in the Sequence

Examine the given nucleotide sequence: 5'-CTGGAATCTCACTTTATCTGG-3'. Identify the histidine codon 'CAC'. It appears as `5'-CTGGAA[TCT CAC] TTTATCTGG-3'`.

Step 3: Plan the Mutation

The target His codon is 'CAC', which needs to be changed to 'CCC' for proline. The sequence segment containing 'CAC' is replaced to create a mutation, forming 'CTGGAATCT[CCC]TTATCTGG'.

Step 4: Design the Mutagenic Oligonucleotide

An oligonucleotide for site-directed mutagenesis should complement and bind to the target region of the DNA strand. Design a 21-nucleotide sequence including the mutation: '5'-CTGGAATCTCCCTTTATCTGG-3'.

Key Concepts

Enzyme Mechanism

The mechanism of enzymes is central to their biological function. Enzymes act as catalysts, speeding up reactions by lowering the activation energy.
Within the enzyme, there are specific regions known as active sites. These are where substrates bind, triggering a reaction. In our exercise, the enzyme's mechanism involves a critical histidine residue within the active site.
Swapping one amino acid for another—such as histidine for proline—alters the enzyme's characteristics.

• Histidine's side chain can accept or donate protons, playing a significant role in catalytic activity.
• Proline, on the other hand, presents a rigid cyclic structure, which can disrupt the folding and flexibility of the enzyme.

Understanding these substitutions helps in identifying how enzyme mechanisms are affected by mutations.
This is a critical insight when studying diseases or designing new enzymes for industrial applications.

Codon Change

In genetic coding, a codon consists of three nucleotides that signal the addition of specific amino acids to a growing polypeptide chain.
For instance, the codon CAC signals for histidine. In the exercise, the task is to change CAC to CCC, coding for proline.
This is called a point mutation, which alters a single nucleotide pair.

• CAC (histidine) involves the nucleotides cytosine, adenine, and cytosine.
• CCC (proline) requires cytosine, cytosine, and cytosine.

A codon change like this can have a profound impact on protein structure and function.
In the context of site-directed mutagenesis, these changes are meticulously planned to study enzyme functions or to generate proteins with new properties.

Oligonucleotide Design

Oligonucleotides are short sequences of nucleotides, designed to match specific sequences on a DNA strand.
In site-directed mutagenesis, they are leveraged to introduce precise mutations into a DNA sequence.
The design includes identifying the sequence of interest and modifying it to include the desired mutation.

• The oligonucleotide must be complementary to the DNA strand, with the mutation embedded within it.
• For a successful binding and mutation, the oligonucleotide typically spans at least 21 base pairs.

In our exercise, the original sequence 5'-CTGGAATCTCACTTTATCTGG-3' is altered using a 21-mer oligonucleotide with the sequence: '5'-CTGGAATCTCCCTTTATCTGG-3'.
This change ensures the correct placement of the proline codon, CCC, in replacement of the histidine codon, CAC.
Precisely designed oligonucleotides are key in genetic engineering, allowing researchers to explore functions and behaviors of specific genes.