The starter grant award of the Chemistry Division to the State University of New York at Buffalo supports the research of Professor Steven T. Diver. The objective of the research is to identify new catalysts by an in vivo screening method. The enzyme system is transketolase with thiamin pyrophosphate cofactor as its catalytic machinery. Through mutation of the transketolase gene in yeast, a system is developed in which yeast grow only when catalytic function is recovered by incorporation of a new cofactor. Various thiamin analogs are designed, synthesized, and tested for activity as new catalytic cofactors. The growth phenotype is used to discover a new cofactor catalyst as an in vivo selection strategy. The research explores a new method for novel catalyst discovery, using a living cell to communicate a reaction's worthiness. The work enhances understanding of the relationship between structure and function in enzyme systems.