Decades of research have led to the development of a numerous high throughput libraries and technologies for screening in yeast. In combination with the most resent advances in synthetic biology, yeast cells can be manipulated to serve as a “factory” for producing a desired product or as a tool to study cellular pathways. We evaluated whether we could express an entire human metabolic pathway in yeast. We chose the purine biosynthesis pathway as our first working model. It is a highly conserved pathway from yeast to humans. In humans there are >20 disorders associated with the pathway that have a variety of symptoms. Importantly, most of these diseases lack any established treatment. However, in some cases the evidence linking mutations in the gene to disorders is rather thin. We propose to use yeast for expressing human mutant alleles of human diseases in order to study whether these mutations actually impact the ability to biosynthesize adenine or other purines and if so, their effect on the cell’s entire metabolic network and for screening for possible treatments. Using a synthetic biology approach, we are swapping the entire purine biosynthesis network of the yeast with the cognate human genes. We have engineered a yeast strain “humanized” for the full de novo adenine biosynthesis pathway. We deleted all of the yeast genes involved in the pathway and complemented them using a neochromosome expressing the human reading frames under the transcriptional control of their cognate yeast promoters and terminators. The “humanized” yeast strain shows growth in the absence of adenine, indicating complementation of the yeast pathway by the full set of human proteins. While the strain with the neochromosome is indeed prototrophic, it grows slowly in the absence of adenine. Dissection of the phenotype revealed that the human ortholog of ADE4, PPAT, shows only partial complementation. We have used several strategies to understand this phenotype, pointing to a possible fundamental difference between the human and the yeast pathways. We also introduced mutations from patients into the human genes to examine their effect on the “humanized” strain. Surprising results suggest that certain missense mutations presumed to underlie human disease conditions have no detectable impact on adenine biosynthesis in yeast. This could be explained by either a second function for the affected protein or an incorrect interpretation of a mutation. Other mutations recapitulate metabolic aspects of the known human disorder LND. Finally, the purine metabolic network can serve as a proof of principle for our ability to take human diseases and the associated pathway/gene networks and establish new yeast models for human disease.