Current Status of the Application of Antimicrobial Peptides and Their Conjugated Derivatives

Abstract

A significant issue in healthcare is the growing prevalence of antibiotic-resistant strains. Therefore, it is necessary to develop strategies for discovering new antibacterial compounds, either by identifying natural products or by designing semisynthetic or synthetic compounds with this property. In this context, a great deal of research has recently been carried out on antimicrobial peptides (AMPs), which are natural, amphipathic, low-molecular-weight molecules that act by altering the cell surface and/or interfering with cellular activities essential for life. Progress is also being made in developing strategies to enhance the activity of these compounds through their association with other molecules. In addition to identifying AMPs, it is essential to ensure that they maintain their integrity after passing through the digestive tract and exhibit adequate activity against their targets. Significant advances are being made in relation to analyzing various types of conjugates and carrier systems, such as nanoparticles, vesicles, hydrogels, and carbon nanotubes, among others. In this work, we review the current knowledge of different types of AMPs, their mechanisms of action, and strategies to improve performance.

Keywords: antibiotics; antimicrobial peptide conjugates; antimicrobial peptide nanostructures; antimicrobial peptides; antimicrobial resistance.

Figure 1

Main mechanisms of antibiotic resistance. Microorganisms can develop several strategies to become resistant to antibiotics. These strategies include the following: (1) Modifying pharmacological targets, such as peptidoglycan or enzymes (e.g., penicillin-binding proteins, DNA gyrase, and DNA topoisomerase). (2) Alterations in membrane permeability due to changes in porin levels, functionality, or selectivity. (3) Active extraction of antibiotics from cells by pumps such as MSF (major facilitator superfamily) and MATE (multidrug and toxic compound extrusion family). (4) Inactivation of enzymes involved in hydrolysis, functional group transfer, or redox processes. (5) Biofilm formation, in which a protective matrix, primarily composed of polysaccharides and proteins, prevents the host from developing immunological mechanisms. (6) The acquisition of resistance genes in the form of free DNA, transposons, plasmids, or integrons through transformation, transduction, or conjugation.

Figure 2

Distribution of Natural AMPs by Origin [21].

Figure 3

Classification of AMPs according to some of the different criteria used by APD3 [21].

Figure 4

Structures of several AMPs taken from the PDB. (a) Antimicrobial peptide RP-1 bound to SDS micelles (18 aa, α-helix L2-R16) [43]; (b) Antimicrobial peptide Arenicin-2 dimer in DPC micelles (two chains with 21 aa every one; chain A: β-sheet formed by three β-strands C3-I10, V13-V15 and R18-C20, and a β-turn R11-G12 stabilized by a disulfide bridge C3-C20; chain B: β-sheet formed by two strands 2W-I10, V13-W20 and a β-turn R11-G12 stabilized by a disulfide bridge C3-C20. The dimer is stabilized by seven intermolecular hydrogen bonds) [44]; (c) Drosophila Peptidoglycan Recognition Protein (PGRP)-SA (180 aa, α-helix L16-G21, S49-E68, S121-Q238, S151-V153, G160-Q168; β-sheet: I14-K15, I35-H42, F78-I80, V86-E88, G106-F111, L141-A149) [45]; (d) Temporin L in solution (13 aa, unstructured) [46].

Figure 5

Main mechanisms of action of AMPs. There are two types of strategies: membrane-directed (a) and non-membrane-directed (b). The former involves interaction with and lysis of the membrane, either through the formation of transmembrane pores (barrel-stave and toroidal-pore models) or without (carpet and aggregate models). The text provides more details about each of these mechanisms. Among the non-membrane-directed mechanisms, one can distinguish between cell wall targeting, whereby AMP binds to components or precursors of this structure (e.g., lipid II), and intracellular targeting, whereby AMP integration results in negative effects on essential processes, such as replication, transcription, translation, enzymatic activities, and protein folding.

Figure 6

(a) Molecular hybrid formed from two active AMPs; (b) a covalent conjugate of two peptides, one of which is an AMP and the other is a targeting peptide. The peptides can bind directly or through a linker, which may or may not be cleaved. (c–e) Covalent conjugates of an active AMP with a fragment (PEG, lipid, or sugar) that lacks antimicrobial activity and acts as an adjuvant. The AMP binds to the PEG and the fatty acid via an amide bond at the N-terminal end. The sugar also forms an amide bond with the side chain of an asparagine (N) residue.

Figure 7

Different formulations of AMPs for improving their antimicrobial action by particular mechanisms. From the information described in Asif et al. [112].