Enzymes are chemical catalysts par excellence. Mechanistic understanding of enzyme catalysis therefore emphasizes two key features—the kinetic pathway and the chemical route taken to achieve it. Elucidation of kinetic mechanisms was elaborated in several chapters earlier (in Part III). Chemical tools and cofactor reactivity exploited by these catalysts formed the major focus of this section (Part IV). Assimilating the kinetic and the chemical line of inquiry completes the comprehension of enzyme function in vitro. The origins of enzyme catalytic power (see Chap. 4 ) make better sense through this fusion. There may be multiple chemical solutions for catalyzing a particular reaction—for instance—peptide bond hydrolysis (Chap. 5 ). A chosen chemical mechanism may place constraints as to what kind of kinetic schemes are feasible. Besides, different kinetic mechanisms may exist for the same enzyme catalyzed reaction. We will attempt to bring together these two broad lines of evidence in this chapter through examples.