Once the primary structure has been created, a secondary structure of some sort evolves.
Parallel or antiparallel, this particular kind of secondary structure is referred to as beta sheet structure. These strands, of course, are not completely flat as shown on this screen. The amide bond portion is flat but the molecules bend on both sides of the alpha-carbon with tetrahedral angles. Consequently they fold up and down and these structures are quite often called beta pleated sheets.
Secondary Structure Mixes
Proteins generally have a mix of different kinds of secondary structure. They are rarely, to my knowledge, all of a particular type. One part of a particular protein may have an alpha helix, whereas another part can be folded back in the beta sheet arrangement.
Whatever type of secondary structure is involved in a particular protein, the secondary structure is held in place by hydrogen bonding interactions between the amino group of the amino acid residue and the carbonyl group of another amino acid residue.
Secondary struture results in the side groups of the amino acid chain of the protein chain sticking out to the side. Certain kinds of side groups will enhance the formation of alpha helix, others will enhance the formation of beta sheet and so on. But whatever the secondary structure happens to be, the side groups are going to be sticking out.
Next we'll take a look at the tertiary structure of proteins which involves the interaction between the side groups.
E-mail instructor: Sue Eggling
Clackamas Community College