What are the common methods for protein purification?

Purification is the crucial first step in any protein-specific study. Four of the most common methods used for protein purification include:

1. Extraction

This process using a variety of methods to break the tissues or cells to free the target protein within and bring it into solution. Common extraction methods include homogenization by high pressure, repeated freezing and thawing, permeabilization by organic solvents, or sonication. The preferred method used depends on the sturdiness of the cells and the fragility of the target protein. The soluble protein that is in the solvent can then be separated from the other components by centrifugation.

2. Precipitation

In this method, increasing amounts of ammonium sulphate (NH4)2SO4 are added to isolate proteins via precipitation. The different fractions of precipitate proteins are collected. Water-soluble proteins are typically the first to be purified with this method. Precipitation is a cost-effective technique for purifying large volumes of protein.

3. Centrifugation

In this protein purification process, centrifugal force is used to separate mixtures of varying densities suspended in a liquid. When a mixture of proteins and other particulate matter is subjected to centrifugation, the high-speed rotation or spinning movement causes the smaller, denser particles to move outward faster than the less dense particles that tend to have more drag in the liquid. 

4. Chromatographic Methods

Chromatography is one of the most widely used protein purification techniques. It works by separating substances based on the differences in their properties. In this method, the solution containing the protein of interest is passed through a column filled with various materials. Different proteins interact with the column materials differently, allowing the target protein to be extracted as it is coming off the column. There are several different chromatography techniques used for protein purification including Column, Affinity, Size Exclusion, Immunoaffinity, and Ion Exchange Chromatography.

The preferred protein purification process depends on the nature of both the protein and the solution as well as the degree of purity required for the intended end use of the protein.