Figure: 22.1 Title: Table 22.1. Examples of the many functions of proteins in biological systems. Caption: The functions of proteins are described.

Figure: 22-00-10UN Title: D-Glyceraldehyde and L-glyceraldehyde. Caption: Naturally occurring monosaccharides have the D-configuration. Naturally occurring amino acids have the L-configuration.

Figure: 22-00-15UN Title: Structures of amino acids at different pH values. Caption: At a low pH value, the amino group and the carboxyl group are protonated. At a high pH value, the amino group is not protonated and the carboxyl group has lost a proton.

Figure: 22-00-16UN Title: Histidine at different pH values. Caption: The structure of histidine depends upon the pH value.

Figure: 22-05-05UN Title: Tripeptides. Caption: A tripeptide is formed from three amino acids.

Figure: 22-05-07UN Title: Resonance contributors in a peptide bond. Caption: The trans configuration is more stable.

Figure: 22-06-01UN Title: A thiol is oxidized to a disulfide. Caption: A common oxidizing agent is bromine or iodine in a basic solution.

Figure: 22-06-02UN Title: Mechanism for the oxidation of a thiol to a disulfide. Caption: The acidic hydrogen is removed by base. The nucleophilic sulfur attacks bromine. Another sulfide group attacks the sulfur, expelling the bromide. A disulfide is formed.

Figure: 22-06-03UN Title: A disulfide can be reduced to a thiol. Caption: The number of S-H bonds increases in a reduction reaction.

Figure: 22-06-04UN Title: Cysteine is oxidized to cystine. Caption: A disulfide bridge is formed.

Figure: 22.6 Title: Figure 22.6. A segment of a polypeptide chain. Caption: Colored squares indicate the plane defined by each peptide bond.

Figure: 22.7 Title: Figure 22.7. Disulfide bridges cross-linking portions of a peptide. Caption: Disulfide bridges contribute to the overall shape of the protein.

Figure: 22-07-01UN Title: Intrachain and interchain disulfide bridges. Caption: Intrachain disulfide bridges bond together different parts of the same polypeptide molecule. Interchain disulfide bridges bond together different polypeptide molecules.

Figure: 22-07-04UN Title: Bradykinin, vasopressin, and oxytocin. Caption: Bradykinin, vasopressin, and oxytocin are peptide hormones.

Figure: 22-07-05UN Title: Aspartame. Caption: Aspartame is Nutrasweet. Aspartame is the methyl ester of a dipeptide of L-aspartate and L-phenylalanine.

Figure: 22-07-07UN Title: Combinations of glycine and alanine. Caption: Four possible dipeptides can be formed from glycine and alanine.

Figure: 22-07-38UN Title: Secondary structure of proteins. Caption: Secondary structure includes hydrogen bonding between peptide bonds.

Figure: 22.8a Title: Figure 22.8a. A segment of a protein in an a-helix. Caption: Each hydrogen attached to an amide nitrogen is hydrogen bonded to a carbonyl oxygen of an amino acid four amino acids away.

Figure: 22.8b Title: Figure 22.8b. The helix is stabilized by hydrogen bonding between the peptide groups. Caption: Each hydrogen attached to an amide nitrogen is hydrogen bonded to a carbonyl oxygen of an amino acid four amino acids away.

Figure: 22.8c Title: Figure 22.8c. Looking at the longitudinal axis of an -helix. Caption: Each hydrogen attached to an amide nitrogen is hydrogen bonded to a carbonyl oxygen of an amino acid four amino acids away.

Figure: 22.9 Title: Figure 22.9. Segments of b-pleated sheets drawn to illustrate their pleated character. Caption: Note that the first is parallel and the second is antiparallel.

Figure: 22.10 Title: Figure 22.10. The backbone secondary structure of carboxypeptidase A. Caption: a-Helical sections are purple. b-Sheet sections are indicated by flat green arrows pointing toward the carboxy terminus of the protein.

Figure: 22.11 Title: Figure 22.11. Three-dimensional structure of carboxypeptidase A. Caption: The tertiary structure of a protein is the three-dimensional arrangement of all the atoms in the protein.

Figure: 22.12 Title: Figure 22.11. Stabilizing interactions involved in maintaining the tertiary structure of a protein. Caption: Tertiary structure is maintained by interactions between side chains of different amino acids within the same protein molecule.

Figure: 22-12-01UN Title: Possible quaternary structure for a hexamer. Caption: The quaternary structure describes the way the subunits are arranged in space.

Figure: 22.13 Title: Figure 22.13. A representation of the quaternary structure of hemoglobin generated by computer graphics. Caption: The orange and green represent the polypeptide chains.