![]() The R group on lysine is frequently chemically modified in order for it to make unusual linkages to other chemical groups or to take part in specific chemical reactions. In proteins, histidine frequently has important roles participating directly in reactions involving hydrogen ion transfer. This means it can pick up or donate hydrogen ions in response to small changes in pH. Histidine contains a nitrogen-containing imidazole functional group that has a pKa of 6. ![]() ![]() In addition, surgical trauma, sepsis, and burns increase demand for arginine and proper healing can require dietary intake. They are frequently called basic, but also are often drawn in their protonated state which is more prevalent at physiological pH.Īrginine (Arg/R) is interesting due to the fact it is an essential dietary amino acid for premature infants, who cannot synthesize it. The variable group in each of these amino acids contains nitrogen, which imparts to the group the ability to exist in protonated and deprotonated states. Included in this group of amino acids are: Basic amino acids (Nitrogen-containing side chains) Those foods frequently contain protein that has broken down to some degree: cooked meats, fermented sauces like Worcestershire or soy, tahini, broths, and yeast extracts. It also is recognized by a receptor in our mouths, contributing to a taste sensation described as “umami.” Many foods contain appreciable amounts of glutamate that are recognized by our taste receptors, and encourage us to eat these substances. In addition to its role as a building block in proteins, glutamic acid (with the deprotonated form named “glutamate”) is a neurotransmitter. It is easy to be confused if they are drawn in this state, because their names include “acid” while the structure shows no ionizable proton and the charge on the R group is negative. At physiological pH, these groups exist primarily in their deprotonated state. These amino acids each contain a carboxylic acid group as part of the variable group. This can lead to the deterioration and loss of teeth. One symptom of the vitamin C deficiency syndrome ‘scurvy’ is the reduced quality of collagen in tissues, including the skin and gums. This occurs in collagen with the aid of ascorbic acid (Vitamin C). Additionally, proline can undergo hydroxylation reactions, stabilizing the protein structure. Proline is exceptional in that it has an R group that folds back and covalently bonds to the backbone of the amino acid, creating a more rigid element in a protein chain that reduces free movement of the polypeptide chain. The small R groups here are more readily packed into tight formations. In proteins that embed themselves into or through membranes, these amino acids can orient themselves toward hydrophobic portions of the inside of the membrane. When incorporated into globular proteins they tend to pack inward among other hydrophobic groups. The amino acids in this group have nonpolar, hydrophobic R groups. Instead, with only very minor exceptions, every amino acid found in cells and in proteins is in the L configuration. Nature has not distributed the stereoisomers of amino acids equally. The designations used in organic chemistry are not generally applied to amino acid nomenclature, but a similar system uses L and D to describe these enantiomers. With the exception of glycine, which has an R-group consisting of a hydrogen atom, all of the amino acids in proteins have four different groups attached to them and consequently can exist in two mirror isomeric forms. The α carbon, carboxylic acid, and amino groups are common to all amino acids, so the R-group is the only variable feature. At the center of each amino acid is a carbon called the α carbon and attached to it are four groups – a hydrogen, a carboxylic acid group, an amine group, and an R-group, sometimes referred to as a variable group or side chain. “It is one of the more striking generalizations of biochemistry …that the twenty amino acids and the four bases, are, with minor reservations, the same throughout Nature.” – Francis CrickĪll amino acids have the same basic structure, shown in Figure 2.1.
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