Organic Chemistry

990 CHAPTER 23 Amino Acids, Peptides, and Proteins

3-D Molecule:

An a-helix

hydrogen bonded to a carbonyl oxygen of an amino acid four residues away. The sub-

stituents on the of the amino acids protrude outward from the helix, thereby

minimizing steric hindrance. Because the amino acids have the L-configuration, the

is a right-handed helix; that is, it rotates in a clockwise direction as it spirals

down. Each turn of the helix contains 3.6 amino acid residues, and the repeat distance

of the helix is 5.4Å.

Not all amino acids are able to fit into an A proline residue, for exam-

ple, forces a bend in a helix because the bond between the proline nitrogen and the

cannot rotate to enable it to fit readily into a helix. Similarly, two adja-

cent amino acids that have more than one substituent on a (valine,

isoleucine, or threonine) cannot fit into a helix because of steric crowding between

the R groups. Finally, two adjacent amino acids with like-charged substituents

cannot fit into a helix because of electrostatic repulsion between the R groups. The

percentage of amino acid residues coiled into an varies from protein to

protein, but, on average, about 25% of the residues in globular proteins are in

-Pleated Sheet

The second type of secondary structure is the In a sheet,

the polypeptide backbone is extended in a zigzag structure resembling a series of

pleats. A sheet is almost fully extended—the average two-residue repeat

distance is 7.0Å. The hydrogen bonding in a sheet occurs between neigh-

boring peptide chains. The adjacent hydrogen-bonded peptide chains can run in the

same direction or in opposite directions. In a parallel sheet, the adjacent

chains run in the same direction. In an antiparallel sheet, the adjacent

chains run in opposite directions (Figure 23.9).

B-pleated

B-pleated

b-pleated

b-pleated

B-pleated sheet. b-pleated

B

a-helices.

a-helix

b-carbon

a-carbon

a-helix.

a-helix

a-carbons

3-D Molecule:

Antiparallel sheetb-pleated

R CH

H N

C

R CH

N

H N

C

R

H

R

HC HC R

O C

HC

R CH

H N

C

R CH

N

H N

C

O C

HC

O

O

H

R

O

O

Parallel

N-terminal N-terminal

C-terminal C-terminal

R CH

H N

C

R CH

N

H N

C

R

H

R

HC R CH

R CH

O C

HC

HC R

H N

R

C

CH

N

H N

C

O C

O

O

H

O

O

Antiparallel

N-terminal

N-terminal C-terminal

C-terminal

Figure 23.9N

Segment of a sheet

drawn to illustrate its pleated

character.

b-pleated

Because the substituents (R) on the of the amino acids on adjacent chains

are close to each other, the chains can nestle closely together to maximize hydrogen-

bonding interactions only if the substituents are small. Silk, for example, a protein

with a large number of relatively small amino acids (glycine and alanine), has large

segments of sheets. The number of side-by-side strands in a sheet

ranges from 2 to 15 in a globular protein. The average strand in a sheet sec-

tion of a globular protein contains six amino acid residues.

Wool and the fibrous protein of muscle are examples of proteins with secondary

structures that are almost all Consequently, these proteins can be stretched.

In contrast, the secondary structures of silk and spider webs are predominantly

sheets. Because the sheet is a fully extended structure, these pro-

teins cannot be stretched.

b-pleated b-pleated

a-helices.

b-pleated

b-pleated b-pleated

a-carbons

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