Sulfur containing mino
Figure 2.14 Amino Acids All amino acids have one feature in common—a carboxyl group and an amino group bonded to the same carbon atom.This carbon atom is also bonded to a side chain (shaded). In solution, the —COOH group is ionized to —COO: and the —NH2 group to —NH3+ giving a net charge of zero to the amino acid.The basic and acidic amino acids have a net positive or negative charge, respectively.The three-letter code name for each amino acid is given.
Nester-Anderson-Roberts: I I. Life and Death of I 2. The Molecules of Life I © The McGraw-Hill
Microbiology, A Human Microorganisms Companies, 2003
Perspective, Fourth Edition properties of the protein, its shape, and how it interacts with other proteins inside the cell. Amino acids that contain many methyl (CH3) groups are non-polar and therefore do not interact with water molecules. Thus, they are poorly soluble in water and are termed hydrophobic (means "water-fearing"). These amino acids tend to be on the inside of protein molecules. Other amino acids contain polar side chains, which make them more soluble in water. They usually occur on the surface of protein molecules.
Some amino acids have side chains with carboxyl or amino groups and they readily form ions which give a net negative (COO:) or positive (NH3+) charge to the amino acid. The former are acidic amino acids (aspartic and glutamic acids) and the latter, basic amino acids (lysine, arginine, and histidine) (see figure 2.14). They are readily soluble in water because the charged water molecules can hydrogen bond to the positive or negative charges of the amino acids. Thus, they are termed hydrophilic (means "water-loving"). Other amino acids have distinctive features that confer important biological functions. For example, the sulfur-containing amino acid cysteine can form covalent bonds with other cysteine molecules in the same protein. The unusual structure of proline allows it to form kinks in the proteins and helps to determine how the protein folds.
All amino acids except glycine can exist in two stereoisomeric forms, a D (right-handed) or L (left-handed) form. Each is a mirror image of the other (figure 2.15; see Glimpse of History). Only L-amino acids occur in proteins, and accordingly, they are designated the natural amino acids. D-amino
Mirror image of left hand
Figure 2.15 Mirror Images (Stereoisomers) of an Amino Acid The joining of a carbon atom to four different groups leads to asymmetry in the molecule. The molecule can exist in either the L- or D- form, each being the mirror image of the other.There is no way that the two molecules can be rotated in space to give two identical molecules.
acids, the unnatural amino acids, are rare in nature and are found in only a few compounds mostly associated with bacteria. They are found primarily in the cell walls and in certain antimicrobial medications termed antibiotics that many bacteria produce. The bacterium Bacillus anthracis, which causes the disease anthrax and has been used as an agent of bioterrorism in the United States, has an outer coat of D-glutamic acid.
Amino acids making up proteins are held together by peptide bonds, a unique type of covalent linkage formed when the carboxyl group of one amino acid reacts with the amino group of another amino acid, with the release of water (dehydration synthesis) (figure 2.16).
The chain of amino acids formed when a large number of amino acids are joined by peptide bonds is called a polypeptide chain. A protein is a long polypeptide chain. One end of the chain has a free amino (—NH2) group, which is termed the N terminal, or amino terminal, end. The other end has a free carboxyl (—COOH) group, which is termed the C terminal, or carboxyl terminal, end. Proteins are always synthesized in cells starting from the N terminal end. Some proteins consist of a single polypeptide chain, whereas others consist of one or more chains joined together by weak bonds. Sometimes, the chains are identical; in other cases, they are different. One or more polypeptide chains make up the protein. In proteins that consist of several chains, the individual polypeptide chains generally do not have biological activity by themselves. Proteins vary greatly in size, but an average-size protein consists of a single polypeptide chain of about 400 amino acids. ■ protein synthesis, p. 175
Proteins have four levels of structure: primary, secondary, tertiary, and quaternary. The number and arrangement or sequence of amino acids in the polypeptide chain determines its primary structure (figure 2.17a). The primary structure in large part determines the other features of the protein. Some amino acids are especially important to functioning of the protein and the substitution of such a critical amino acid with another often destroys the ability of the protein to carry out its function. This will be discussed in greater detail in chapter 8.
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