Glycans have several distinct properties that make their development as disease biomarkers appealing. Firstly, their location on cell surfaces makes them the first point of contact for cellular interactions, and thus they are crucial in the control of normal metabolic processes, and conversely, they function as pathogen adhesion receptors. Secondly, specific glycan structures that are not present, or are in low amounts in normal state, proliferate or alter their sequence in disease states. And, lastly, changes in glycosylation may be found in many proteins, including those that are highly abundant. Thus changes in the normal levels of glycan structures, such as terminal sialic acid, may be markers of disease states. New highly-specific reagents are required in order to overcome current limitations in the discovery and exploitation of disease-related glycans. Using structurally-guided genetic manipulations, we will convert the NanB sialidase from S. pneumococcus into a high-specificity affinity reagent for the detection of sialic acid modifications of glycopeptides and glycoproteins. Because such a protein has lectin-like properties, but is derived from an enzyme, it is called a Lectenz(r). A NanB Lectenz(r) addresses a key need in disease glycomarker detection: namely, a robust and easy to produce reagent specific for ?3-linked sialic acid. This reagent could be employed in an affinity matrix for sample enrichment, which in conjunction with existing MS based methods could provide linkage information. It could also be employed in histological studies, or Western blots, etc. Lectenz(r) offer numerous advantages over plant lectins: they are engineered to be high affinity and yet retain the exquisite substrate specificityof the endogenous enzyme, they may be efficiently produced, and for human homologues have the potential to be employed in vivo with low toxicity. Whereas some aspects of Lectenz(r) development parallel those of antibody evolution, Lectenz(r) have the tremendous benefit of employing a protein naive template that has the desired specificity.
Using structurally-guided genetic manipulations we will convert the NanB carbohydrate-processing enzyme into a high-specificity affinity reagent (called a Lectenz(r)) for the detection of disease-related sialic acid modifications of glycopeptides and glycoproteins. The principle advantages of engineered Lectenz(r) over other reagents, such as antibodies or lectins, is that they have exquisite substrate specificity that is not context dependent, they may be evolved to have desirable binding kinetics, and that they may be efficiently produced as monomeric proteins.
