###### Abstract
In their review, [PERSON] and [PERSON] [31] discuss the effect of high (up to hundreds MPa) hydrostatic pressure on various types of enzymes (oxidoreductase, transferases, hydrolases, and lyases). These authors emphasize that in general, high pressure can help to stabilize enzymes against thermal denaturation, but at the same time, the enzymatic activity can be either increased or suppressed depending on each specific case. [PERSON] et al. [32] and [PERSON] et al. [33] demonstrated that the sensitivity to high pressure is the individual characteristic of each enzyme. In experiments performed by [PERSON] et al., the enzymatic activity of polygalacturonase decreased rapidly (down to complete inactivation after 10 min treatment) upon the action of high (up to 500 MPa) pressure, while pectinethylesterase retained ~70% of its initial enzymatic activity even after 20 min treatment at 800 MPa [32]. [PERSON] et al. observed a decrease in \(\beta\)-glucosidase and lipase activities at 0.1 MPa and higher pressures, while \(\alpha\)-amylase exhibited higher activity at high (400 MPa) pressure; these authors also demonstrated pressure-induced activation of lumbrokinase at 200 MPa and higher (up to 500 MPa) pressures [33]. Once again, in the above-mentioned papers [6,30], studies of the pressure impact on proteins are limited to the cases with constant pressure. In this regard, studying the influence of fast rise-time-pulsed pressure impacts on proteins represents an actual direction of research. Peroxidase enzyme systems are known to perform important functions in various metabolic processes. Peroxidase pertain to heme-containing enzymes, which are well represented in plant and animal tissues [34]. This provokes a great interest in studying this class of enzymes. In humans, myeloperoxidase participates in atherogenesis [8] and in oxidative stress [10]. For these considerations, a peroxidase has been employed in our present study as a model object. Namely, horseradish peroxidase, which is comprehensively characterized in the literature, is often employed as a model object in studies of a wide class of peroxididases. HRP represents a glycoprotein with a molecular weight of about 40 to 44 kDa [35,36], which can form aggregates [37]. HRP catalyzes the oxidation of many organic and inorganic compounds by hydrogen peroxide [38]. It is known that HRP represents a D-isomer, whose structure includes 77% \(\alpha\)-helices and 12% \(\beta\)-sheets [39]; its macromolecule also includes 18% to 27% of carbohydrate chains, which stabilize the protein structure [36,40,41]. Accordingly, the structure of this enzyme is chiral (or more exactly, pseudo-chiral). AFM allows one to visualize and measure the functional activity of single enzyme molecules [42; 43], thus representing a very convenient tool for the determination of their aggregation state. This is quite useful for single-molecule enzymology. For this reason, in our research, atomic force microscopy (AFM) was employed to study the effect of external factors on HRP aggregation. Previously, AFM was employed to study the formation of various protein complexes [44; 45; 46; 47; 48; 49; 50; 51; 52; 53]. In parallel, in our present study, a commonly used spectrophotometry-based technique was employed to estimate the enzymatic activity of HRP in solution. Our present work is aimed at the AFM investigation of the influence of short (subnanosecond and nanosecond) electromagnetic (640 kV/m) and pressure (10 atm) pulses on protein aggregation. The pulse rise-time was ~200 ps in the case of PEMF, and 80 ns in the case of the pulsed pressure field. In the literature, such pulses were called ultra-short electromagnetic pulses (USEMP) [54]. The parameters of the electromagnetic field have been chosen based on the above-discussed factors, since such electromagnetic field characteristics are interesting from a practical point of view for diagnostic applications (for instance, for microwave imaging of cancer [4; 5]).