Metabolic Engineering has enjoyed tremendous successes in the development of highly productive microorganisms for a variety of products of interest, especially the spectrum of chemical structures collectively referred to as natural products. Implementation of the tools and techniques of the field have been applied most often to improve small molecule productivity in a native organism, or to transfer the pathway(s) of interest from one or more source organisms into a more tractable host. Less represented are examples of novel pathway design for the optimized production of a desired target molecule. We seek to design and construct functional biosynthetic pathways for non-natural and/or non-native products based only on the specification of a target compound and knowledge of the biotransformation potential encompassed in enzymes. This "retrobiosynthetic design" approach relies on the tremendous advances in directed enzyme evolution to propose enzymatic conversions by considerations of only the functional groups involved. We present a novel pathway design for 3-hydroxybutyrolactone, a compound with no identified natural source. Considerations for the design of this pathway as well as progress-to-date in its construction will be discussed. Additionally, a simple database tool under development to facilitate pathway design will be presented.