This paper presents a set of investigations on the binding of a metabolic inhibitor, azide with prosthetic heme group of biomolecules, hemin chloride (Hem) and cytochrome c (Cyt c) immobilized on Au and Ag nanoparticles. A variety of spectroscopic tools have been used to understand the chemistry occurring on the nanoparticle surface. While the nature of binding of the model system, hemin has been investigated by UV - visible, fluorescence, FTIR, and Raman spectroscopies, the azide binding has been studied in detail by MALDI-TOF MS. Hemin binding on the nanoparticle surface occurs through the carboxylic acid groups. The hemin - N3 adduct on the nanoparticle surface has been detected by mass spectrometry and its fragments have been studied by post source decay analysis. The chemistry of hemin on the nanoparticle surface has been compared with that of the protein, Cyt c. Azide binding of Cyt c requires thermal activation due to reduced accessibility of the heme center, unlike in the case of hemin. The binding chemistry is similar for free Cyt c and Cyt c bound to the nanoparticles. © 2005 American Chemical Society.