Amino acids are the molecular building blocks of proteins. Every amino acid has a central carbon atom, called the α-carbon, bonded to two functional groups: an ammonium group (NH₃⁺) (shown on the left side of the carbon) and a carboxylate group (COO⁻) (shown on the right side of the carbon). The α-carbon is also bonded to a hydrogen atom (shown below the carbon) and a side chain called an R group (shown above the carbon). There are many amino acids in nature; however, there are only 20 amino acids commonly found in the proteins of living organisms. The type of amino acid can be classified according to the specific R group. There are 9 nonpolar R groups that are hydrophobic. Valine is an example of a nonpolar amino acid. The alpha carbon is circled in blue and the hydrophobic R group is circled in black, in a structure where the carboxylate group is to the left of the alpha carbon, the ammonium group is below the alpha carbon, and the R group is to the right of the alpha carbon. The remaining biologically significant amino acids are all polar and are divided into three different groups. Polar neutral amino acids contain oxygen or sulfur atoms but carry no charge. An example of this type of amino acid is threonine, which has a hydroxyl group in the R group. Another class of amino acid is polar acidic, which contain a carboxylate group in the R group and carry a negative charge at physiological pH. An example of this class of amino acid is aspartate. Finally, there are 3 polar basic amino acids, which contain an ammonium group in the R group and carry a positive charge at physiological pH. An example of this class of amino acid is lysine. Classify the following amino acid, which has a CH₂CH₂C＝₂O(NH₂) for the R group. Is it a, b, c or d? The correct answer is (b). This is the structure of glutamine. The alpha carbon is circled in blue. The R chain, circled in black, contains the neutral or uncharged amide group. At physiological pH, the functional groups on the alpha carbon are ionized. Let’s review these two functional groups. Amines are Brønsted–Lowry bases because the lone pair of electrons on the nitrogen atom accepts H⁺ from water. If methylamine is put in a solution of water, the ammonium ion, methylammonium ion, is produced. Likewise, the amine functional group in amino acids is an ammonium ion at physiological pH. The second group attached to the alpha carbon is the carboxylic acid group. An important property of carboxylic acids is their dissociation in water. When a carboxylic acid, such as propanoic acid, dissociates in water, an H⁺ is transferred to a water molecule to form a negatively charged propanoate ion, and a positively charged hydronium (H₃O⁺). Likewise, the carboxylic acid functional group in amino acids is a carboxylate ion at physiological pH. Complete the following statement about functional groups in amino acids: In an amino acid at physiological pH, the a, b, c or d? The correct answer is (b). At physiological pH, the carboxylic acid group loses a proton to form a carboxylate ion with a negative charge. Amino acids are the building blocks for physiological compounds. Serotonin helps us to relax, sleep deeply and peacefully, think rationally, and gives us a feeling of well-being and calmness. Serotonin is synthesized from the amino acid tryptophan, shown, as the precursor to serotonin and carbon dioxide being produced, which can cross the blood–brain barrier. A diet that contains foods such as eggs, fish, cheese, turkey, chicken, and beef, which have high levels of tryptophan, will increase serotonin levels. Foods with a low level of tryptophan, such as whole wheat, will lower serotonin levels. Here is another example of using amino acids as building blocks for physiological compounds. Histamine (along with carbon dioxide) is synthesized in the nerve cells in the hypothalamus from the amino acid histidine, with the assistance of histidine decarboxylase enzyme. Histamine is produced by the immune system in response to pathogens and invaders, or injury. When histamine combines with histamine receptors, it causes allergic reactions, which may include inflammation, watery eyes, itchy skin, and hay fever. Identify the structure of cysteine at physiological pH. Is it a, b, c or d? The correct answer is (a). At physiological pH, the amino acid cysteine contains an ammonium group and a carboxylate group.