Delphinidin

The cells of living organisms contain from several hundred to many thousands of different kinds of proteins, which are second only to cellulose in making up the dry weight of plant cells. Each kind of organism has a unique combination of proteins that give it distinctive characteristics. There are, for example, hundreds of kinds of grasses, all of which have certain proteins in common and other proteins that make one grass different from another. The hundreds of kinds of daisies are distinguished from each other and from grasses by their particular combinations of proteins.

Analysis of proteins helps evolutionary botanists sort out relationships and heredity among plants and is a popular, current area of research. Proteins consist of carbon, hydrogen, oxygen, and nitrogen atoms, and sometimes also sulfur atoms. Proteins regulate chemical reactions in cells, and comprise the bulk of protoplasm apart from water. Protein molecules are usually very large and consist of one or more polypeptide chains with, in some instances, simple sugars or other smaller molecules attached.

Polypeptides are chains of amino acids. There are 20 different kinds of amino acids, and from 50 to 50,000 or more of them are present in various combinations in each protein molecule. Each amino acid has two special groups of atoms plus an R group. One functional amino acid group is called the amino group (—NH2); the other, which is acidic, is called the carboxyl group (—COOH). The structure of an R group can vary from a single hydrogen atom to a complex ring. Some R groups are polar, while others are not, and each is distinctive for one of the 20 amino acids. Glycine is representative of general amino acid structure.

Amino acids are linked together by peptide bonds, which are covalent bonds formed between the carboxyl carbon of one amino acid and the nitrogen of the amino group of another, a molecule of water being removed in the process. Plants can synthesize amino acids they need from raw materials in their cells, but animals have to supplement from plant sources some amino acids they need, since they can manufacture only a few amino acids themselves.

Each polypeptide usually coils, bends, and folds in a specific fashion within a protein, which characteristically has three levels of structure and sometimes four:

1. A linear sequence of amino acids fastened together by peptide bonds forms the primary structure of a protein.

2. As hydrogen bonds form between oxygen atoms of carboxyl groups and hydrogen atoms of amino groups in different molecules, the polypeptide chain can coil to form a spiral-like staircase, called an alpha helix. The helix is one version of secondary structures that may form. Other secondary structures include polypeptide chains that double back and form hydrogen bonds between two lengths in what is referred to as a beta sheet, or pleated sheet.

3. Tertiary structure develops as the polypeptide further coils and folds. The tertiary structure is maintained by interactions and bonds among R groups.

4. If a protein is composed of more than one kind of polypeptide, a fourth, or quaternary structure, forms when the polypeptides associate. The three-dimensional structure of a protein may be somewhat flexible in solution, but chemicals or anything that disturbs the normal pattern of bonds between parts of the protein molecule can denature the protein. Denaturing alters the characteristic coiling and folding and adversely affects the protein’s function and properties.

Denaturing may be reversible, but if it is brought about by high temperatures or harsh chemicals, it may kill the cell of which the protein is a part. For example, boiling an egg, which is mostly protein, brings about an irreversible denaturing; the solid egg proteins simply can’t be restored to their original semiliquid condition.