## 3 Brief Overview of Proteolytic Enzymes: Bromelain, Ficin, and Papain's Classification  
Proteolytic enzymes, also referred to as proteases or peptidases, form a crucial class of enzymes that catalyze the hydrolysis of peptide bonds in proteins. This process leads to the breakdown of proteins into their smaller building blocks, namely monopeptides or even amino acids. More than 4000 peptidases are known, with around 700 found in humans, and they are extensively used across many industries, including food (i.e., in cheese making, where rennet induces milk proteins' coagulation and transformation into curd), dairy, meat, bakery, brewing, feed, detergent, leather, and pharmaceutical ones [28, 29, 30].  
Proteolytic enzymes belong to the class of hydrolases and can be classified in a variety of ways. Considering their chemical mechanism of catalysis, proteolytic enzymes are divided into serine proteinases, aspartic proteinases, threonine peptidases, metalloproteinases, and cysteine proteinases--the latter category includes bromelain, ficin, and papain, our enzymes of focus [28, 30, 31, 32]. Furthermore, proteolytic enzymes are categorized by their mode of action into exopeptides and endopeptidase (proteinases). Exopeptides cleave peptide bonds near the amino or carboxyl termini of the substrate, generating multiple peptides, depending on the cleavage site. If the exopeptides act on the N-terminus, they produce a single amino acid (aminophenedases), or a di- or tripeptide, while exopeptides targeting the C-terminus increase the production of a tripeptide or dipeptide (carboxypeptidases). Dipeptidase and omega-peptidase exopeptides may occur as well. Endopeptidase, on the other hand, hydrolyze chemical bonds located within the peptide chain. Combining these two categories, an enzyme can be, for example, a serine endopeptidase or a serine exopeptidase, etc., [28, 30, 31, 32]. Bromelain, ficin, and papain are cysteine endopeptidase. More specifically, bromelain cleaves peptide bonds between aromatic, basic, and hydrophobic residues [33], ficin cleaves bonds in proteins like casein or gelatin [34], and papain owns a broad specificity [34]. Moreover, the protease source, which is mainly plant (where bromelain, ficin, and papain belong), animal, or microbial, and their distinct biological purposes, such as wound healing, defense mechanism, anti-diabetic, anti-inflammatory, anti-melanoma, antioxidant, skin renewal, and skin sounding action, are secondary ways of proteolytic enzyme classification that further highlight their importance in cosmetic products and formulations [31]. All the aforementioned categories are illustrated in Figure 2.  
The following analysis focuses on the biochemical properties, enzymatic activity, and mechanisms of action of these enzymes. Further investigations concern their sources, extraction, and purification methods, as well as factors influencing their stability and efficacy. Their applications in cosmetic formulations are also explored, highlighting their role in exfoliation, skin renewal, and general skin care, along with sustainability considerations and their potential for use in environmentally friendly cosmetic products.