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For example, the amino acid glycine has a hydrogen atom as the R group. The chemical nature of the side chain determines the amino acid's nature (that is, whether it is acidic, basic, polar, or nonpolar). The R group (or side chain) is different for every amino acid. For example, the letter V or the three-letter symbol val represent valine. A single uppercase letter or a three-letter abbreviation represents amino acids. Scientists use the name "amino acid" because these acids contain both amino groups and carboxylic acid groups in their basic structure.
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Variation in the amino acid sequence is responsible for the enormous variation in protein structure and function. There are 20 common amino acids present in proteins, each with a different R group. Every amino acid also has another atom or group of atoms bonded to the central atom known as the R group. Each amino acid has the same fundamental structure, which consists of a central carbon atom, or the alpha ( α) carbon, bonded to an amino group (NH 2), a carboxyl group (COOH), and to a hydrogen atom. They are hydrophilic and contain various polar functional groups: hydroxyl groups in serine and threonine, a sulfhydryl group in cysteine, and amide groups in asparagine and glutamine.Īmino acids are the monomers that comprise proteins. The polar uncharged amino acids are serine, threonine, cysteine, asparagine, and glutamine. They have amino groups on their side chains that accept a proton to become positively charged. The basic amino acids are lysine, arginine, and histidine. They have carboxyl groups that donate a proton to become negatively charged. The acidic amino acids are aspartate and glutamate. The aromatic amino acids are phenylalanine, tyrosine, and tryptophan, all of which are large and capable of participating in hydrophobic interactions. The amino acids with aliphatic R groups are glycine, the simplest amino acid alanine, valine, leucine, and isoleucine, which are extremely hydrophobic and associate with each other to form stable core structures in proteins methionine, a sulfur-containing amino acid and proline, which has an aliphatic ring.Īromatic groups are hydrocarbons that exist as unsaturated rings with alternating single and double bonds. Polar amino acids can be divided into acidic, basic, and uncharged at a neutral pH.Īliphatic groups are single bonded hydrocarbons that exist as straight, branched, or cyclic chains. Non-polar amino acids can be divided into aliphatic and aromatic. Amino acids are broadly classified into two categories based on their R-groups: non-polar and polar. At physiological pH, amino acids usually exist as dual-charged and are called zwitterions, molecules with an equal number of positive and negative charges.Įvery amino acid has a unique R group that is responsible for their varied features that include their size, solubility, and charge. The amine group on an amino acid can act as a base and accept a proton resulting in a positive charge, whereas the carboxyl group can act as an acid and donate a proton resulting in a negative charge. Amino acids used in protein synthesis are always L-isomers. These isomers are mirror images that are not superimposable on each other. These four groups can form two distinct configurations that are stereoisomers, called L and D isomers. As glycine has a hydrogen atom as its R group, it does not have four distinct groups and therefore does not have a chiral center. It is attached to an amino group, a carboxyl group, a hydrogen atom, and a variable R group.Ī chiral molecule has four distinct groups arranged around a particular atom, known as the chiral center. These twenty amino acids are often referred to by their three or one-letter abbreviations.Īll amino acids have a central carbon called the alpha carbon, which functions as a chiral center for most amino acids. Although there are hundreds of amino acids, there are twenty that function as building blocks of proteins. Amino acids are organic compounds that are predominantly made up of carbon, hydrogen, oxygen, and nitrogen.