氨基酸

The alpha carbon in amino acids is bonded to four distinct groups: an amino group, a carboxyl group, a hydrogen atom, and a variable R group. These four groups arrange around the alpha carbon, which functions as a chiral center for most amino acids. The R group differs for each amino acid, determining its unique chemical properties and characteristics.

Glycine is the simplest amino acid because its R group is just a hydrogen atom, not a more complex side chain. Since it has only three distinct groups attached to its alpha carbon instead of four, glycine lacks a chiral center. This makes glycine unique among the twenty common amino acids used in protein synthesis.

A zwitterion is a molecule with an equal number of positive and negative charges. At physiological pH, amino acids exist as zwitterions because the amino group accepts a proton and becomes positively charged, while the carboxyl group donates a proton and becomes negatively charged. This dual-charged state is the typical form of amino acids in living cells.

Nonpolar amino acids have hydrophobic R groups that are either aliphatic or aromatic hydrocarbons. Polar amino acids have hydrophilic R groups containing functional groups like hydroxyl, sulfhydryl, or amide groups. The chemical nature of the R group determines whether an amino acid is acidic, basic, polar, or nonpolar, directly affecting its behavior in proteins.

Aromatic amino acids like phenylalanine, tyrosine, and tryptophan have R groups containing unsaturated rings with alternating single and double bonds. Aliphatic amino acids like glycine, alanine, and valine have single-bonded hydrocarbon R groups arranged as straight, branched, or cyclic chains. Both types are nonpolar and hydrophobic, but aromatic amino acids are larger and participate in hydrophobic interactions.

Acidic amino acids like aspartate and glutamate have carboxyl groups on their R groups that donate protons, becoming negatively charged. Basic amino acids like lysine, arginine, and histidine have amino groups on their side chains that accept protons, becoming positively charged. These charged R groups enable amino acids to participate in ionic interactions and affect protein structure and function.

Amino acids exist as L and D isomers, which are stereoisomers and mirror images that cannot be superimposed on each other. Proteins exclusively use L-isomers because biological systems evolved to recognize and incorporate only this configuration during protein synthesis. This specificity ensures consistent protein structure and function across all living organisms.