Answer:

To purify a GST-tagged fusion protein by affinity chromatography, the ligand used is glutathione (option D). This is because GST (glutathione S-transferase) is commonly used as a fusion tag in recombinant protein expression systems, and it has a high affinity for glutathione.

Explanation:

Affinity chromatography is a powerful technique used to purify proteins based on their specific binding interactions with ligands immobilized on a solid support. In the case of GST-tagged fusion proteins, the GST tag can be exploited for purification using a glutathione-based affinity chromatography method.

Glutathione:
Glutathione is a tripeptide composed of glutamate, cysteine, and glycine. The GST tag consists of the GST protein, which has a high affinity for glutathione. The binding interaction between glutathione and the GST tag is stable and specific, allowing for effective purification of GST-tagged fusion proteins.

Procedure:
The purification process involves the following steps:

1. Immobilization: Glutathione is covalently attached to a solid support, such as a chromatography resin or a column matrix. This immobilized glutathione will serve as the ligand for affinity chromatography.

2. Sample loading: The sample containing the GST-tagged fusion protein is applied to the column or resin containing the immobilized glutathione.

3. Binding: The GST tag binds specifically and tightly to the immobilized glutathione through the formation of a stable complex.

4. Washing: Non-specifically bound proteins and contaminants are washed away using a buffer solution. The GST-tagged fusion protein remains tightly bound to the glutathione ligand.

5. Elution: The GST-tagged fusion protein is then eluted from the column or resin by adding a solution containing excess free glutathione. The high concentration of free glutathione competes with the immobilized glutathione, disrupting the binding interaction and releasing the purified protein.

6. Collection: The eluted protein is collected and can be further analyzed or used for downstream applications.

Conclusion:
In conclusion, the ligand used in affinity chromatography to purify a GST-tagged fusion protein is glutathione. The strong and specific binding between glutathione and the GST tag allows for efficient purification of the fusion protein from a complex mixture of proteins.