Final answer:
Amino acids with positively charged side chains like lysine, arginine, and sometimes histidine, are limited to structural positions within proteins where their positive charge facilitates key interactions, such as binding to negatively charged molecules or within enzymatic active sites.
Step-by-step explanation:
Amino Acids with Positively Charged Side Chains
Amino acids with positively charged side chains, such as lysine and arginine, are known as basic amino acids. These amino acids are limited to positions where the interaction with negatively charged molecules is required, like in the active sites of enzymes or when binding to DNA, which has a negatively charged phosphate backbone. Basic amino acids often play crucial roles in molecular recognition and signal transduction processes. Another amino acid, histidine, although not always positively charged at physiological pH, can become positively charged under certain conditions, allowing it to act similarly to lysine and arginine in specific contexts.
The location of these amino acids is not random but is often crucial to the structure and function of proteins. In summary, amino acids with positively charged side chains are limited to positions in proteins where their unique properties can be utilized, such as in electrostatic interactions and binding sites that require a positive charge.