USA Biology Olympiad (USABO) Practice Exam

What is a likely consequence of replacing a lysine with a glycine in a protein?

1. Loss of a negatively charged side chain

2. Increased binding affinity for substrates

3. Altered protein folding and stability

4. Improved enzymatic activity

The correct answer is: Loss of a negatively charged side chain

The replacement of lysine with glycine in a protein typically leads to altered protein folding and stability. Lysine has a positively charged side chain due to its amino group, which can participate in various interactions, including ionic bonds and hydrogen bonds. This characteristic plays a critical role in maintaining the protein's structure and function. On the other hand, glycine is the smallest amino acid with a simple hydrogen side chain, providing flexibility without the charge. This replacement can disrupt existing interactions crucial for maintaining the protein's conformation, potentially leading to changes in folding and overall stability. Depending on the context of the protein and its function, this alteration can have significant consequences in biological processes. Whereas one could consider the aspect of electrostatic interactions by pointing out that a positively charged side chain is lost with the substitution of glycine, the more pronounced effect is usually on the protein's folding and stability. Thus, the changes in properties and interactions due to this modification make altered protein folding and stability a more compelling outcome.