The relationship between the molecular conformation of Nisin and its antibacterial activity

Nisin's molecular conformation is the core basis for its antimicrobial activity. Composed of 34 amino acids, it contains 5 lanthionine rings linked by thioether bonds, as well as special structures such as a hinge region and flexible N-terminal/C-terminal regions. Minor conformational differences exist between natural variants, and all these conformational features are closely associated with antimicrobial activity. The specific relationships are as follows:

Core Cyclic Structures: Key Support for Antimicrobial Activity

The 5 lanthionine rings are the core structures maintaining Nisin's antimicrobial activity. The first 3 rings (Rings a, b, c) and the tightly connected last 2 rings (Rings d, e) are separated by a 3-residue hinge region.

On one hand, the N-terminal Rings a and b can form a cage-like complex with lipid Ⅱ in the bacterial cell wall—an important initial binding step for Nisin's antimicrobial action, which inhibits bacterial cell wall synthesis.

On the other hand, the integrity of Ring c is crucial for antimicrobial activity; its destruction or removal essentially abolishes Nisin's antimicrobial efficacy.

Additionally, these cyclic structures stabilize Nisin's overall conformation through cross-linking, enabling it to maintain structural stability in common food processing environments (e.g., acidic conditions) and thus sustain antimicrobial activity.

Flexibility of the Hinge Region: Ensuring Efficient Antimicrobial Action

The hinge region connecting the two sets of cyclic structures exhibits excellent flexibility, a conformational feature that provides versatility for Nisin's interactions with bacterial cell membranes and lipid Ⅱ.

After the N-terminal rings bind to lipid Ⅱ, the flexible hinge region allows the C-terminal Rings d and e to better insert into the bacterial phospholipid bilayer.

Reduced flexibility in this region—for example, limiting amino acid residue mobility via mutation—weakens Nisin's interaction with the bacterial membrane lipid layer, thereby decreasing antimicrobial activity.

Meanwhile, the hinge region can adapt to lipid molecules of different conformations, expanding Nisin's spectrum of action against various Gram-positive bacteria.

Conformational Characteristics of N/C Termini: Influencing Efficacy and Stability

Both the N-terminal and C-terminal regions of Nisin possess certain conformational flexibility, and they perform distinct roles in the antimicrobial process.

The N-terminal 1–22 amino acid residues can penetrate the lipid phase of the bacterial cell membrane, laying the foundation for subsequent membrane structure disruption.

The C-terminal region binds to anionic lipids on the bacterial cell membrane via ionic interactions, assisting Nisin in pore formation.

Furthermore, the integrity of the C-terminus significantly affects antimicrobial activity: removing the C-terminal 5 residues reduces Nisin's efficacy by approximately 10-fold, while further removing 9 residues (including two lanthionine rings) decreases activity by 100-fold.

Minor Conformational Differences in Natural Variants: Leading to Divergent Antimicrobial Activity

Nisin has multiple natural variants (e.g., Nisin A, Nisin Z) with highly similar overall conformations. However, minor conformational changes caused by local amino acid differences result in variations in antimicrobial activity and application properties.

For example, Nisin A and Nisin Z differ only at the 27th amino acid (histidine in Nisin A vs. asparagine in Nisin Z). This subtle structural difference makes Nisin Z more water-soluble, enabling better dispersion in food matrices, and its overall antimicrobial activity is superior to Nisin A.

Exceptions exist: Nisin A has a lower minimum inhibitory concentration (MIC) against Clostridium spp., making it more effective for inhibiting such bacteria.

Synergistic Association Between Aggregated Conformation and Antimicrobial Activity

Active Nisin molecules often exist as dimers or tetramers; this aggregated conformation is critical for efficiently disrupting bacterial cell membranes.

Aggregation of multiple Nisin molecules allows the formation of transmembrane pores in the bacterial cell membrane, leading to the leakage of key intracellular substances (e.g., ATP) and ultimately bacterial death.

If environmental factors (e.g., extreme pH) cause disaggregation into monomers, Nisin retains some antimicrobial activity but exhibits significantly reduced efficiency in transmembrane pore formation, resulting in a marked decline in overall antimicrobial capacity.