In even slightly acidic conditions protonation of the nitrogen occurs, changing the properties of histidine and the polypeptide as a whole. As too much flexibility is usually not desired, as a structural component it is less common than alanine. It is able to fit into the tightest spaces, e.g., the triple helix of collagen. It is the smallest amino acid, rotates easily, adds flexibility to the protein chain. Like isoleucine, leucine and valine, these are hydrophobic and tend to orient towards the interior of the folded protein molecule.īecause of the two hydrogen atoms at the α carbon, glycine is not optically active. Phenylalanine, tyrosine, and tryptophan contain large rigid aromatic group on the side chain. Has longer, slightly more flexible side chain.Įssential for humans. When located inside of the protein, aspartate and glutamate are usually paired with arginine and lysine.īehaves similar to aspartic acid. Binds to positively-charged molecules and ions, often used in enzymes to fix the metal ion. Usually is located on the outer surface of the protein, making it water-soluble. Carries a hydrophilic acidic group with strong negative charge. insulin).īehaves similarly to glutamic acid. Disulfides are also found in peptides too small to hold a stable shape on their own (eg. When cystines are part of a protein, insulin for example, this stabilises tertiary structure and makes the protein more resistant to denaturation disulfide bridges are therefore common in proteins that have to function in harsh environments including digestive enzymes (e.g., pepsin and chymotrypsin) and structural proteins (e.g., keratin). Under oxidizing conditions, two cysteines can join together in a disulfide bond to form the amino acid cystine. The sulfur atom binds readily to heavy metal ions. It behaves fairly neutrally, can be located in both hydrophilic regions on the protein outside and the hydrophobic areas inside. More stiff than glycine, but small enough to pose only small steric limits for the protein conformation. Conditionally essential amino acids are not normally required in the diet, but must be supplied exogenously to specific populations that do not synthesize it in adequate amounts. ‡ An essential amino acid cannot be synthesized in humans and must, therefore, be supplied in the diet. † The stop codon is not an amino acid, but is included for completeness. * UGA is normally a stop codon, but encodes selenocysteine if a SECIS element is present. Gene expression and biochemistry Amino Acid
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Protein pKa calculations are sometimes used to calculate the change in the pKa value of an amino acid in this situation. Note: The pKa values of amino acids are typically slightly different when the amino acid is inside a protein. IUPAC/IUBMB now also recommends that Sec or U refers to selenocysteine, and Pyl or O refers to pyrrolysine. The three-letter symbol Asx or one-letter symbol B means the amino acid is either asparagine or aspartic acid Glx or Z means either glutamic acid or glutamine and Xle or J means either leucine or isoleucine. The one-letter symbol for an undetermined amino acid is X. Note that forming a peptide bond results in elimination of a molecule of water, so the mass of an amino acid unit within a protein chain is reduced by 18.01524 Da. The masses listed are based on weighted averages of the elemental isotopes at their natural abundances. Structures and symbols of the 20 amino acids which are directly encoded for protein synthesis by the standard genetic code.įollowing is a table listing the one-letter symbols, the three-letter symbols, and the chemical properties of the side chains of the standard amino acids.