Читать книгу Cell Biology - Stephen R. Bolsover - Страница 74

Proteins are made up of individual amino acid building blocks. Amino acids contain both a carboxyl group, which readily gives an H⁺ to water and is therefore acidic, and a basic amino group (‐NH₂), which readily accepts H⁺ to become ‐NH₃ ⁺. Figure 3.7a shows two amino acids, leucine and γ‐aminobutyric acid( GABA), in the form in which they are found at normal pH: the carboxyl groups have each lost an H⁺ and the amino groups have each gained one, so that the molecules bear both a negative and a positive charge.

We name organic acids by labeling the carbon adjacent to the carboxyl group α, the next one β, and so on. When we add an amino group, making an amino acid, we state the letter of the carbon to which the amino group is attached. Hence leucine is an α‐amino acid while GABA stands for gamma‐aminobutyric acid. α‐Amino acids are the building blocks of proteins. They have the general structure shown in Figure 3.7b where R is the side chain. Leucine has a simple side chain of carbon and hydrogen. Other amino acids have different side chains and so have different properties. It is the diversity of amino side chains that give proteins their characteristic properties (page 104).

α‐Amino acids can link together to form long chains through the formation of a peptide bond between the carboxyl group of one amino acid and the amino group of the next. Figure 3.7c shows the generalized structure of such a chain of α‐amino acids. If there are fewer than about 50 amino acids in a polymer we tend to call it a peptide. More and it is a polypeptide. Polypeptides that fold into a specific shape are proteins.