From Chain to D Structure

The DNA-determined sequence of amino acids in proteins is called their primary structure (1) (A). This sequence contains all the information about the further behavior of the respective amino acid chain. The side chains (R-groups) of the individual amino acids predict the spatial arrangement of the protein. Today, the primary structure of even complex proteins is known. The first amino acid sequence discovered was that of insulin (1952). Insulin is a comparatively simple protein, consisting of two chains, one 21 (A-chain) and the other 30 (B-chain) amino acids long. The pancreas first synthesizes proinsulin, a single-chain protein that contains an additional 33 amino acids (C-peptide). Then, in a posttrans-lational rearrangement, the above-mentioned two chains are joined into the finished protein, while the C-pep-tide is removed.

helices, three of which combine into a stable triple helix. Switches in chain orientation by 180° require a so-called p-loop, consisting of four amino acids with a hydrogen bond between amino acids 1 and 4. Globular proteins usually contain all of those secondary structures. Insulin, for example, is made of 57% a-helices, 6% pleated sheets, and 10% p-loops, with the remaining 27% structured otherwise.

The tertiary structure results from the arrangement of the folded chains in space (3) that is due to interactions between the amino acid side chains (R-groups). For instance, in insulin, three disulfide bridges between pairs of cysteine R-groups essentially determine the molecule's spatial arrangement. Proteins have biological activity only as long as their tertiary structure is maintained. Denaturation by ethanol, heat, or acids, for instance, causes loss of function, which may or may not be reversible.

The folding of the amino acid chain (2) determines its secondary structure. Folding is achieved through the formation of hydrogen bonds between the -C=O and the HN- groups involved in the peptide bonds. Certain amino acids (e. g., tyrosine, valine, isoleucine) favor the formation of a pleated sheet structure, the result of several chains combined in a parallel or counter-parallel arrangement; its peptide grid folds like a harmonica with side chains sticking out below and above. Another energetically favorable secondary structure is the helix. In a right-handed a-helix, the peptide shape spirals clockwise in such a way that each turn includes approximately 3.6 amino acid R-groups. The collagen of the connective tissue matrix, for instance, consists of left-handed

Quarternary structures result from an assembly of several tertiary structures. Such oligomers, consisting of several subunits or monomers, are common in large, globular proteins. The monomers may have independent biological activity, whereas some become functional units only inside a quaternary structure (e.g., enzyme complexes). Insulin, too, forms quarternary structures: in blood, some of it occurs as dimers. A pancreatic storage form consists of Zn2+-stabi-lized hexamers. Those are used as slow-release insulins in diabetes therapy.

i- A. From Chain to 3-D Structure

NH e

1 Primary structure

CC H NH

CC H NH

.Ri2

1 Primary structure

Tertiary Structure Insulin
3 Tertiary structure

Structural proteins

4 Quarternary structure

Globular proteins

4 Quarternary structure

Common Monomers
Two monomers
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