Recently, a collaborative research team led by Professor Yong-Qiang Li of Shandong University and Professor Yanmei Yang of Shandong Normal University systematically investigated the physical binding mechanisms between enzymes and the photosensitizer chlorin e6 (Ce6), and proposed a catalytically enhanced strategy for photodynamic therapy (PDT). The study demonstrated that the extent of positively charged regions on the enzyme surface could be served as a reliable indicator for evaluating and predicting the enzyme’s binding affinity to Ce6. Based on this criterion, the authors further developed catalase-Ce6 nanoconjugates (CAT-Ce6 NCs) exhibiting excellent stability and potent photodynamic antibacterial activity. The CAT-Ce6 NCs effectively remodeled hypoxic pathological microenvironments and eradicated bacteria, thereby promoting the advancement of catalysis-augmented PDT of bacterial infections. The aforementioned study, titled "Deciphering the Physical Binding Mechanism of Enzyme–Photosensitizer Facilitates Catalysis-Augmented Photodynamic Therapy," was published in the journal Research.

Research Background
Photodynamic therapy (PDT), characterized by its non-invasive nature and precise spatiotemporal selectivity, has garnered considerable attention for its applications in tumor and anti-infection therapies. However, the clinical translation of PDT continues to encounter several challenges. Firstly, traditional photosensitizers (PS) frequently display poor solubility and insufficient stability. Additionally, the complex pathological microenvironment can hinder PS delivery and release, while tissue hypoxia markedly inhibits the generation of reactive oxygen species (ROS), ultimately leading to therapeutic failure. Enzyme–PS conjugates offer a promising strategy to address these limitations. Enzyme carriers can improve the physicochemical properties of PS, while their biocatalytic activity facilitates the remodeling of the pathological microenvironment, thereby enabling catalysis-augmented PDT. However, conventional covalent conjugation methods are prone to compromising enzyme activity and may lead to undesirable side effects. In contrast, physical binding approaches not only better preserve enzyme activity but also simplify the operational procedures. However, the physical binding mechanisms between enzymes and PS, which encompass various weak forces, are complex and remain insufficiently explored. Therefore, a comprehensive understanding of the physical binding mechanisms between enzymes and PS will offer valuable insights for the rational design of enzyme–PS conjugates, thereby opening new avenues for the development of catalysis-augmented PDT.

Research Progress
Through the integration of molecular dynamics (MD) simulations and experimental studies, this study systematically investigated the interaction mechanisms between three representative enzymes—lysozyme (Lys), glucose oxidase (GOx), and catalase (CAT)—and the chlorin e6 (Ce6) (Figure 1). The results demonstrated that the initial binding of Ce6 to enzymes was primarily driven by electrostatic interactions, leading to the formation of a loose complex that was subsequently stabilized by hydrophobic interactions and hydrogen bonds (Figure 2).

Through free energy calculations and residue analysis of enzyme–Ce6 binding sites, the results demonstrated that positively charged and hydrophobic residues on the enzyme surface was critical determinants of enzyme–Ce6 binding (Figure 3). Furthermore, the inclusion of three enzymes—horseradish peroxidase (HRP), alkaline phosphatase (ALP), and lactate dehydrogenase A (LDHA)—which exhibited substantial differences in molecular weight and possessed significant biomedical relevance, further corroborated these findings.

By examining the relationship between the binding affinity of Ce6 for enzymes and the surface areas of positively charged and hydrophobic residues, the authors demonstrated that the surface area of positively charged regions on the enzyme was strongly correlated with its binding affinity. Considering intravenous administration, this study further incorporated human serum albumin (HSA) and established a systematic criterion for assessing the binding strength between enzymes and Ce6 (Figure 4).

Based on this criterion, the authors further developed catalase (CAT)-Ce6 nanoconjugates (CAT-Ce6 NCs) that exhibited excellent stability and high biocompatibility (Figure 5). In vitro experiments demonstrated that CAT-Ce6 NCs efficiently catalyze the conversion of excess H₂O₂ to O₂ in the bacterial microenvironment, thereby alleviating hypoxia and enhancing the production of ROS (Figure 6). In a murine subcutaneous abscess model infected with methicillin-resistant Staphylococcus aureus (MRSA), CAT-Ce6 NCs effectively modulated the hypoxic pathological microenvironment and eradicated the bacteria, thereby enabling catalysis-augmented PDT (Figure 7).

Future Prospects
Through a systematic investigation of the physical interactions between enzymes and Ce6 photosensitizer, this study revealed for the first time that positively charged and hydrophobic residues on the enzyme surface were key structural determinants of their binding. Furthermore, this study proposed that the area of positively charged regions on the enzyme surface can be used as a criterion to evaluate and predict the binding affinity of enzymes and Ce6. Based on this criterion, the authors successfully developed CAT-Ce6 NCs with exceptional stability, which efficiently remodeled the hypoxic pathological microenvironment and completely eradicated pathogens, thereby achieving catalysis-augmented PDT of bacterial infections. This study establishes a theoretical framework to guide the design of physically bonded enzyme–PS conjugates and facilitate catalysis-augmented PDT.

The complete study is accessible Via DOI: https://doi.org/10.34133/research.0732

About Research by Science Partner Journal
Launched in 2018, Research is the first journal in the Science Partner Journal (SPJ) program. Research is published by the American Association for the Advancement of Science (AAAS) in association with Science and Technology Review Publishing House. Research publishes fundamental research in the life and physical sciences as well as important findings or issues in engineering and applied science. The journal publishes original research articles, reviews, perspectives, and editorials. IF = 10.7, Citescore = 13.3.