Notice that the protein chain has a beginning (an N-terminus with a amino group) and an end (a C-terminus with a carboxyl group). Each amino acid is denoted by a three letter code (Asp-Lys-Gln-His-Cys-Arg-Phe). A heptapeptide, Aspartic Acid-Lysine-Glutamine-Histidine-Cysteine-Arginine-Phenylalanine is shown below. Each shows differing amounts of information. There are many different ways to represent the structure of a polypeptide or protein. At some point, when the structure is long enough, it is called a protein. Likewise, we can have tripeptides, tetrapeptides, and other polypeptides. When two amino acids link together to form an amide link, the resulting structure is called a dipeptide. In a reverse reaction, the peptide bond can be cleaved by water (hydrolysis). In this reaction, water is released (condensation). The resulting link between the amino acids is an amide bond which biochemists call a peptide bond. With the exception of the amino acid cysteine (Cys) with a -CH 2SH for an R group (which happens to have an R stereocenter), all of the remaining amino acids found in proteins have an S stereocenter.Īmino acids form polymers when an amino group of an amino acid is covalently attached to the carbonyl carbon (C=O) of the carboxyl group of the next amino acid. All amino acids in proteins have the absolute configuration shown above. All of the other 19 naturally-occurring amino acids have one stereocenter (at the carbon containing the amine and carboxyl groups) and can exist as two possible enantiomers only the L-enantiomer occurs in proteins. The smallest amino acid is glycine (Gly) which has a hydrogen atom as its R group. The R groups are classified as generally nonpolar, polar charged, or polar uncharged. In an amino acid, the central (alpha) carbon has an amine group (RNH 2, RNH 3 +), a carboxylic acid group (RCOOH, RCOO -), a hydrogen (H), and one of twenty different R groups (also called side chains) attached to it. There are twenty different naturally occurring amino acids that differ in one of the four groups connected to a central carbon atom. Proteins are polymers consisting of monomers called amino acids. Humans have about 24,000 different proteins which catalyze chemical reactions, recognize foreign molecules and pathogens, allow cellular and organism movement, and regulate cell response, including cell division and death. Proteins are large biological molecules that have molecular weights ranging from the thousand to the millions.
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