**1. Introduction**

Marine organisms cover three-fourths of the land surface, claiming to be a treasure for many biologically active substances, especially proteins and carbohydrates. The extraction of proteins from marine sources has been increasing at a tremendous rate due to their distinct environment and biological activity. For instance, collagens from fish species have been explored by researchers for many years to sort-out the suitable materials for biomaterial fabrication in tissue engineering applications. It is well-known that collagen, the most abundant structural protein in the extracellular matrix (ECM), is the main component of various connective tissues in the body [1,2]. Based on the structure, molecular composition

**Citation:** Ge, B.; Hou, C.; Bao, B.; Pan, Z.; de Val, J.E.M.S.; Elango, J.; Wu, W. Comparison of Physicochemical and Structural Properties of Acid-Soluble and Pepsin-Soluble Collagens from Blacktip Reef Shark Skin. *Mar. Drugs* **2022**, *20*, 376. https://doi.org/ 10.3390/md20060376

Academic Editor: Sik Yoon

Received: 14 May 2022 Accepted: 31 May 2022 Published: 2 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and distribution, collagens are classified according to at least 29 different types [3]. However, the main backbone of most of the collagens is composed of chains of polypeptides with the repeating sequence (Gly-X-Y)n, where X and Y are commonly occupied by proline and hydroxyproline [1]. Collagen from the skins of several fish species such as tilapia [4], horse mackerel, yellow sea bream, and tiger puffer [5], black ruff (*Centrolophus niger*) [6], and Parang-Parang (*Cirocentrus dorab*) [7] were reported earlier.

Much empirical evidence proved the adaptability of fish collagen for the fabrication of different biomaterials for artificial tissue implants. For instance, different types of scaffolds were fabricated using collagens from tilapia [8,9], *Trachicephalus uranoscopus* [10], bigeye snapper *Priacanthus hamrur* [11], etc. Fish collagen has been used as an alternative to mammalian counterparts for food, biomedical and pharmaceutical uses due to safety reasons and religious constraints [12]. In addition, fish processing industries produce large quantities of byproducts, in which most of the sources such as skin, bones, and scales are poorly utilized [13].

The blacktip reef shark (*Carcharhinus melanopterus*) belongs to the Carcharhinidae family, a species of requiem shark which is commonly found in Pacific regions like southern Asia, the Philippines and northern Australia. Shark is commonly used for shark fin and fillet production. During shark processing, the generated wastes, particularly skin and cartilage, can be ultimately used as potential source material for collagen extraction [14,15]. Unfortunately, none of the studies has used blacktip reef shark skin for collagen extraction and, therefore, it is important to understand the properties of collagens extracted from blacktip reef sharks, due to its potential value. Hence, this study is intended to accomplish the above objectives to extract the collagens using acetic acid and pepsin. It is important to investigate the physicochemical and functional properties of collagens that are extracted in two different ways: with and without enzymatic digestion. Therefore, in the present study, we tried to extract and characterize acid-soluble and pepsin soluble collagen from Blacktip reef shark (*Carcharhinus melanopterus*) skin, and provided a simultaneous comparison of collagens. Accordingly, the present results would be useful in developing a viable alternative collagen material for further biomedical and pharmaceutical applications.
