Why is Arg often referred to as a basic amino acid? Which two other amino acids are also basic amino acids?

Arginine, often symbolized as Arg or R, is one of the twenty standard amino acids required by the human body for protein synthesis. Its uniqueness stems from the guanidinium group present in its side chain, which at physiological pH typically retains a positive charge. This attribute makes Arginine a basic amino acid.

Within proteins, Arginine is known for its versatility in interactions due to this charged guanidinium group. It can form multiple hydrogen bonds, and participate in ionic interactions, playing a critical role in the function of enzymes, signaling pathways, and the maintenance of cellular homeostasis. Arginine is also a precursor for the synthesis of nitric oxide, an important cellular signaling molecule. Due to its basic side chain, Arginine can increase the pH of its environment by accepting protons (H+ ions).

Lysine, designated as Lys or K, is another essential amino acid that is grouped among the basic amino acids. Its side chain ends in an amine group (-NH2) which becomes positively charged at physiological pH. Like Arginine, this charge allows Lysine to play a pivotal role in protein structure and function.

Lysine is crucial for the formation of stable protein structures as it's able to participate in hydrogen bonding and ionic interactions. Additionally, its ability to react with specific receptors on the cell surface and to form crosslinks in certain types of proteins such as collagen, is vital for the structural integrity of tissues. The amine group of Lysine also enables it to serve as a binding site for essential biochemical processes, including the attachment of phosphate groups or the ubiquitination process that tags proteins for degradation.

Histidine, shown as His or H in shorthand, possesses a unique side chain encompassing an imidazole ring—a ring structure capable of both donating and accepting protons. This structural feature allows Histidine to be positively charged under certain circumstances and contributes to its classification as a basic amino acid.

At physiological pH, Histidine is often partially charged and plays crucial roles in the active sites of enzymes, where its buffer-like qualities allow it to stabilize pH fluctuation during enzymatic reactions. Its ability to bind and release protons makes it instrumental in enzyme catalysis. Histidine is also involved in the binding of metal ions and the transmission of signals across proteins, signifying its importance in many physiological processes including the immune response and growth.

Physiological pH refers to the normal operating pH level within the human body, which typically sits around 7.4. This slightly alkaline environment is key for the optimal operation of numerous biological processes, including enzyme activity, oxygen transport by hemoglobin, and electrical activity in neurons.

Basic amino acids, like Arginine, Lysine, and Histidine, have side chains that accept protons due to the presence of nitrogen in their functional groups, having a greater tendency to be positively charged at this pH level. The balance of these charges is essential to maintain the stability and functionality of proteins. Disruptions to the physiological pH can lead to altered charges on these amino acids, potentially impacting protein stability and function, which underscores the crucial nature of maintaining physiological pH within the body's tightly regulated systems.