Modularity - the concept that complex proteins can be broken down into simpler, interdependent components - manifests in enzymatic evolution and underpins their functional diversification. Glycosyltransferases and kinases, enzymes with crucial roles in cellular processes, serve as exemplars to decipher modular evolution. Detailed comparative analysis of these enzymes provides novel insights into their inherent functional plasticity, illuminating versatile allosteric mechanisms to regulate catalysis. This research further extends to enzyme engineering, demonstrating how understanding enzymatic modularity can facilitate the design of new enzymes with desired functions. Ultimately, this dissertation presents a comprehensive framework for the modular understanding of enzymatic evolution and its implications for bioengineering, which can aid in the development of novel therapeutic and biotechnological applications.