As noted in earlier posts such as ‘Biopharmaceutical Protein Structure – Part 1‘, the elaborate 3-dimensional structure of proteins is maintained by a complex interplay of chemical bonds and electrostatic interactions. These electrostatic interactions primarily involve the charged side-groups of the protein’s constituent amino acids. As seen in Figure 1 below, the ‘R‘ groups which represent clusters of atoms unique to each of the 20 known amino acids, protrude away from the central protein chain. The electrically-charged nature of these ‘R‘ groups, allows them to interact with other protruding sub-molecular bodies along the length of the protein chain such as Hydrogen atoms. The interplay between these sub-molecular entities is responsible to a large extent, for maintaining the protein’s 3-dimensional structure, thereby allowing the protein to carry out its function.
If therefore, we place a protein in an environment rich in charged particles such as occurs in solutions of dissolved salts, the electrically-charged salt particles (ions) will interact with the charged side-groups of the protein. This will alter the existing pattern of interactions along the protein chain and as can be expected, will change the charge pattern of the protein. This in turn results in changes to its shape and as mentioned so many times before, a change in a protein’s shape alters its activity or ability to function.
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In the majority of cases, even a slight change in a protein’s shape will lead to a dramatic change in its activity. More often than not, this change in activity is a negative one. In some cases, the altered protein may be partially active or completely inactive and harmless, but in other cases the altered protein may both less active and harmful. This latter scenario is what must be guarded against with greatest vigor. However, predicting alteration in one direction or another is very difficult and so the safest approach is minimizing the possibility of any change at all.
The profound impact of charged particles in solution on a protein’s structure, means that great care must be taken in following the manufacturer’s recommendations for reconstituting and diluting drugs in general and protein drugs, specifically. In practice, if instructed to reconstitute a protein drug in dextrose solution as an example, this should be strictly adhered to and alternative solvents such as saline should be avoided. If not, we risk damaging the protein and with it, exposing patients to ineffective and potentially dangerous treatments.