The Beta Sheet

The second (hence "beta") recognizable regular secondary structure in proteins is the P sheet. The amino acid residues of a P sheet, when viewed edge-on, form a zigzag or pleated pattern in which the R groups of adjacent residues point in opposite directions. Unlike the compact backbone of the a helix, the peptide backbone of the P sheet is highly extended. But like the a helix, P sheets derive much of their stability from hydrogen bonds between the carbonyl oxygens and amide hydrogens of peptide bonds. However, in contrast to the a helix, these bonds are formed with adjacent segments of P sheet (Figure 5-5).

Interacting P sheets can be arranged either to form a parallel P sheet, in which the adjacent segments of the

Figure 5-3. View down the axis of an a helix. The side chains (R) are on the outside of the helix. The van der Waals radii of the atoms are larger than shown here; hence, there is almost no free space inside the helix.

(Slightly modified and reproduced, with permission, from Stryer L: Biochemistry, 3rd ed. Freeman, 1995. Copyright © 1995 by W.H. Freeman and Co.)

Figure 5-3. View down the axis of an a helix. The side chains (R) are on the outside of the helix. The van der Waals radii of the atoms are larger than shown here; hence, there is almost no free space inside the helix.

(Slightly modified and reproduced, with permission, from Stryer L: Biochemistry, 3rd ed. Freeman, 1995. Copyright © 1995 by W.H. Freeman and Co.)

polypeptide chain proceed in the same direction amino to carboxyl, or an antiparallel sheet, in which they proceed in opposite directions (Figure 5-5). Either configuration permits the maximum number of hydrogen bonds between segments, or strands, of the sheet. Most P sheets are not perfectly flat but tend to have a right-handed twist. Clusters of twisted strands of P sheet form the core of many globular proteins (Figure 5-6). Schematic diagrams represent P sheets as arrows that point in the amino to carboxyl terminal direction.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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