Understanding the distinction between non-ionizable and ionizable R groups in amino acids is crucial for grasping their chemical behavior and interactions. Non-ionizable R groups are characterized by their uncharged nature, meaning they do not participate in acid-base reactions or proton transfers. Consequently, amino acids with non-ionizable R groups possess only two pKa values: one for the amino group and one for the carboxyl group, as their R groups do not contribute a third pKa value. There are 13 amino acids that fall into this category, which are unable to form ionic bonds due to their lack of charge.
In contrast, ionizable R groups are capable of gaining a charge through acid-base reactions, which allows them to participate in ionic bonding. Amino acids with ionizable R groups have three pKa values, including one for the R group itself. This ability to ionize is a significant distinction, as it enables these amino acids to engage in various biochemical interactions. At physiological pH (around 7), certain amino acids are positively charged, including arginine, histidine, and lysine, which can be remembered using the mnemonic "dragons eat knights riding horses." These three amino acids are classified as basic and positively charged.
On the other hand, aspartic acid and glutamic acid (or aspartate and glutamate) are negatively charged at physiological pH, represented by the mnemonic "dragons eating." This group consists of two amino acids. Additionally, there are amino acids that are ionizable but remain uncharged at physiological pH, specifically cysteine and tyrosine. Cysteine, with a three-letter code of CYS, has a pKa of 8.3 for its sulfhydryl group. When the pH exceeds 8.3, cysteine can donate a hydrogen ion, resulting in a negative charge. Similarly, tyrosine has a pKa of 10.1, and when the pH is above this value, it can also donate a hydrogen ion and gain a negative charge.
In summary, there are a total of 7 ionizable amino acids, while the remaining 13 amino acids are non-ionizable. Recognizing the ionization behavior of cysteine and tyrosine is essential for understanding their roles in biochemical processes, especially as we delve into more complex topics in biochemistry.