**4. Conclusions**

As only a few peptidases of the S8 family so far have been applied in the preparation of bioactive oligopeptides, there is still a need to develop more S8 serine peptidases with potentials in bioactive oligopeptide preparation. In this study, the peptidase Aa2\_1884 from marine bacterium *Flocculibacter collagenilyticus* SM1988<sup>T</sup> was demonstrated to be a novel multimodular gelatinase of the S8 family, which has high activity towards gelatin and denatured collagens. Moreover, under the optimized enzymolysis conditions, Aa2\_1884 has a high hydrolysis efficiency (95%) on bovine bone collagen. The prepared hydrolysate is rich in collagen oligopeptides and has antioxidant activity. The results in this study sugges<sup>t</sup> that Aa2\_1884 has a promising potential application in preparing collagen oligopeptides from bovine bone collagen. The collagen hydrolysate prepared with Aa2\_1884 may have good bioactivities due to its high oligopeptide content as well as good antioxidant activity, which awaits further investigation.

**Author Contributions:** Conceptualization, X.-L.C. and Y.-Z.Z.; investigation, J.L. and J.-M.D.; methodology, J.L., J.-H.C., M.-L.S., J.-P.W. and X.Z.; software, Z.-J.T.; project administration, X.-L.C. and X.-Y.Z.; resources, X.-L.C. and X.-Y.Z.; supervision, X.-M.T. and X.-Y.Z.; writing—original draft preparation, J.L. and Z.-J.T.; writing—review and editing, X.-L.C. and X.-Y.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the National Science Foundation of China (grant numbers U2006205, U1706207, 31670038 and 31971535, awarded to X.-L.C., Y.-Z.Z., X.-L.C. and Y.-Z.Z., respectively), the National Key R&D Program of China (2018YFC0310704 awarded to X.-L.C.), the Major Scientific and Technological Innovation Project (MSTIP) of Shandong Province (2019JZZY010817 awarded to Y.-Z.Z.), Taishan Scholars Program of Shandong Province (tspd20181203, awarded to Y.-Z.Z.), and Scientific Research Think Tank of Biological Manufacturing Industry in Qingdao (QDSWZK202002 awarded to Y.-Z.Z.).

**Data Availability Statement:** The amino acid sequence of Aa2\_1884 has been submitted to NCBI database under the accession number WP\_199608745, It can be found here: https://www.ncbi.nlm. nih.gov/protein/WP\_199608745 (accessed on 6 December 2021).

**Acknowledgments:** The authors sincerely appreciate Jiaojiao Tian (Ocean University of China) for her assistance in the amino acid analyses. We also thank Jingyao Qu, Jing Zhu, and Zhifeng Li from State Key Laboratory of Microbial Technology, Shandong University, China for their help and guidance in LC-MS.

**Conflicts of Interest:** The authors declare no conflict of interest.
