Passage
Antimicrobial peptides are an important component of innate immunity. Since their discovery in 1939, over 5,000 peptides with antimicrobial properties have been isolated from organisms in every domain of life. They have demonstrated activity against enveloped viruses, bacteria, fungi, and even tumor cells and have recently gained interest for their potential in fighting antibiotic-resistant bacteria. Most known antimicrobial peptides are positively charged, but some with negative charges have also been discovered.Frogs are known to secrete a variety of antimicrobial peptides as a major source of immunity. Secretions from a new species of frog were collected and the peptides were separated by cation exchange chromatography. Four of the isolated peptides demonstrated strong antibacterial activity and were characterized by isoelectric focusing and sequencing:
Figure 1 Results of isoelectric focusing of four antimicrobial peptidesTable 1 Amino Acid Compositions of Four Antimicrobial Peptides
Adapted from Wang Z, Wang G. APD: the Antimicrobial Peptide Database. Nucleic Acids Res. 2004;32(Database issue) :D590-2.
-One of the challenges of antimicrobial peptides is their short physiological half-life. To solve this problem, some researchers have begun investigating synthetic antimicrobial peptides composed of D-amino acids. Why might these peptides have an increased half-life in the body relative to natural antimicrobial peptides?

Question

Passage
Antimicrobial peptides are an important component of innate immunity. Since their discovery in 1939, over 5,000 peptides with antimicrobial properties have been isolated from organisms in every domain of life. They have demonstrated activity against enveloped viruses, bacteria, fungi, and even tumor cells and have recently gained interest for their potential in fighting antibiotic-resistant bacteria. Most known antimicrobial peptides are positively charged, but some with negative charges have also been discovered.Frogs are known to secrete a variety of antimicrobial peptides as a major source of immunity. Secretions from a new species of frog were collected and the peptides were separated by cation exchange chromatography. Four of the isolated peptides demonstrated strong antibacterial activity and were characterized by isoelectric focusing and sequencing:

Figure 1 Results of isoelectric focusing of four antimicrobial peptidesTable 1 Amino Acid Compositions of Four Antimicrobial Peptides

Adapted from Wang Z, Wang G. APD: the Antimicrobial Peptide Database. Nucleic Acids Res. 2004;32(Database issue) :D590-2.
-One of the challenges of antimicrobial peptides is their short physiological half-life. To solve this problem, some researchers have begun investigating synthetic antimicrobial peptides composed of D-amino acids. Why might these peptides have an increased half-life in the body relative to natural antimicrobial peptides?

Quizplus · Accepted Answer

11ecba2d_0df5_a69c_a3bf_b511c4d5b8e8_MD0008_00 With the exception of glycine, which has no chiral center, amino acids are chiral molecules that can exist in two enantiomeric conformations, designated L and D forms. In nature, amino acids are almost always found in the L-conformation, so almost all biomolecules are designed to work with L-amino acids. Proteases typically recognize specific amino acid side chains and cleave the peptide bond nearest that side chain. When D-amino acids are used to make peptides, the side chain protrudes from the peptide at a different angle than it does in the L-conformation and is unrecognized by proteases. D-peptides therefore remain uncleaved and have a longer physiological half-life than their L counterparts. (Choice B) D- and L-amino acids have essentially identical solubility. (Choice C) A peptide bond is an amide bond between two amino acids. Amide bonds require approximately the same amount of energy to break, regardless of the chemical groups on either side of the bond. (Choice D) Hydrolysis of both L- and D-peptides can be catalyzed by strong acids or strong bases. Because physiological pH is nearly neutral, these modes of catalysis are essentially absent in the body. Educational objective: Amino acids have chiral centers and can exist in two conformations, designated L and D. Almost all natural amino acids are synthesized in the L-conformation. As a result, enzymes essentially work exclusively with L-amino acids and do not affect D-peptides.

Passage Antimicrobial Peptides Are an Important Component of Innate Immunity. Since