Antibodies have become increasingly important agents in diagnostics and therapy, and there are growing demands for novel designer molecules that bind to a given site of a protein target. The existing methods allow production of antibodies against a given linear epitope. However, the majority of sites on the surface of a folded protein are conformational epitopes, and currently there are no technical means for obtaining antibodies to a pre-specified conformational epitope. This limitation is an important problem as this impedes development of novel antibody-based therapeutics. We propose a novel approach for engineering antibodies that bind to a pre-specified epitope of a folded protein. We wish to apply this technology for producing inhibitory antibodies to ectoenzymes that play a principal role in tumor progression and metastasis. In particular, we have shown that the cell surface ectonucleotidase CD39 hydrolyzes extracellular nucleotides to produce adenosine, which strongly suppresses anti-tumor immunity and promotes angiogenesis. Inhibition of CD39 with a small-molecule compound, polyoxometalate-1, significantly inhibits tumor growth, but poor selectivity and toxicity limit the therapeutic effects. The objective of thi application is to develop a universal technology for engineering antibodies that bind to a given epitope of a folded protein, and as an example, produce a potent and selective antibody inhibitor of mouse CD39. We hypothesize that inhibitory antibodies to CD39 and other enzymes can be engineered from a common precursor anti-fluorescein antibody by targeting active sites of enzymes. We will test this hypothesis as follows. Aim 1: Establish the inhibitory function of th anti-fluorescein antibody by targeting the active site of CD39. Based on the 3D structure of CD39, we will create artificial antibody-binding sites near the active site of CD39 by introducing cysteine residues via mutagenesis and labeling these with fluorescein. We will then identify a CD39 mutant where enzymatic activity is completely inhibited by the anti-fluorescein antibody, indicating the optimal position of the antibody for blocking the active site. Aim 2: Generate binding complementarity between the inhibitory anti-fluorescein antibody and CD39 independent of fluorescein label. We will use a traditional approach of affinity maturation by randomization of complementarity determining regions of the antibody and selection for improved binding. We expect that selection for binding will preserve inhibitory functions and produce the inhibitory antibody to mouse CD39. We anticipate the following positive impacts: First, the inhibitory antibody to mouse CD39 will allow us to evaluate in subsequent animal studies the full therapeutic potential of targeting CD39 in cancer. Second, the derived technology will enable rational engineering of inhibitory antibodies to human CD39 and other key ectoenzymes implicated in cancer progression. In addition, the developed technology will meet needs in research, diagnostics and fundamentally advance the field of therapeutic antibody engineering.