Amino acids and proteins

Amino acids as the name implies< have properties of both amines and carboxylic acids. The general formula for an amino acid is

+H₃N – CHR – COO^-,

where R can be any side chan. As can be seen from the structure, amino acids are zwitterious, meaning that they have both negative and positive charges at neutral pH.

The simplest amino acid, glycine, has a single hydrogen for its R group. Others, such as alanine and valine, have a long aliphatic chains for their R group. Lysine, arginine and valine, have an additional amino group, and hence are basic. Serine and threonine have aliphatic hydroxyl side chains. Phenylalanine, tyrosine, and tryptophan have aromatic side chains. Asparate and glutamate have an additional carboxylic acid group, and hence are acidic. Asparagine and glutamine have amid side chains. Cysteine and methyonine contain sulfur in their side chain. Often, a one letter symbol is used to identify these amino acids (see table).

Amino acids that cannot be produced by the organism in question, and must be supplied by an external source, are called “essential amino acids.” The table lists the abbreviations, symbols, and R groups for amino acids. Those marked asterisks are essential acids for human.

Joining hundreds and even thousands of amino acids makes protein molecules by peptide and sulfide bonds. Each protein has an unique sequence of amino acids. This sequence of amino acids, along with the location of sulfide bonds, is known as the “primary structure” of the protein. The “second structure” refers to the steric relationship of amino acids that are close to one another in the linear sequence.

“Tertiary structure” refers to the steric relationships of amino acids that are far apart in the linear sequence. Proteins are not straight chains, but these chains are folded, making a three-dimensional structure. The dividing line between secondary and tertiary structure is somewhat arbitrary.

“Quaternary structure” of a protein refers to the way the various peptide chains are packed together. Each chain is referred to as a “subunit.” The “isoelectric point” is the pH at which there is no net electrical charge on a protein. This is a different va-

lue for various proteins. Proteins are important to living things, as they constitute the vast majority of an organism’s structure (fibrous proteins) and enzymes involved in the metabolism (globular proteins).

1. Proteins are the principal structural and regulatory molecules of cells

2. These large polymers are composed of hundreds of amino acids (nitrogen-containing monomers)

a. Every amino acid has the same fundamental structure, consisting of a central carbon bonded to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen, and a variable group represented by the letter R

b. The different R-groups attached to the amino acid determine the specific type of amino acid

c. Peptide bonds are formed when the amino group of one amino acid is linked to the carboxyl group of the next amino acid

3. The same basic set of 20 amino acids is found in nearly all proteins, but the different arrangements and proportions of these amino acids are responsible for the wide variety of proteins found in organisms

4. Proteins form highly organized shapes or conformations, which are extremely important to their function

a. If a protein loses its shape, it loses its function (denaturation)

b. If a protein regains its shape, it may regain its function (renaturation)

5. Biologists recognize four levels of protein organization

a. The primary structure is the sequence of amino acids

b. The secondary structure is the repeated, regular structure assumed by protein chains, which commonly form a helix or pleated sheet

c. The tertiary structure is the complex three-dimensional structure of a single peptide chain

d. The quaternary structure is the complex three-dimensional structure of a protein composed of more than one peptide chain

6. Proteins give structure and form to cells and regulate chemical reactions within cells

7. Enzymes are proteins that regulate chemical reactions within cells by altering the rate of the reactions

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