Electrochemical Deconstruction Along with Spectroscopic Investigations of Electroactive Heme Proteins and Enzymes
Authors: Samajdar Rudra Narayan , Bhattacharyya Aninda Jiban 
Published: February 2020

The iron – porphyrin system, known in biology as heme, is ubiquitous in the chemistry of modern living systems. Heme forms the active site for a large class of biologically relevant molecules including proteins and enzymes like hemoglobin, myoglobin, cytochromes etc. Heme based molecules perform a host of biochemical functions out of which redox homeostasis (controlled by cytochromes) and oxygen storage and transport (controlled by hemoglobin / myoglobin) are some of the most important features. Some of these molecules also serve as good biomarkers for different types of physico-chemical stress on organisms. It is well known that the functionality of these heme based proteins / enzymes is carefully modulated by the oxidation state of iron at the active site. Hence, it is important to understand the redox chemistry of iron in these systems in order to understand their biochemical functions more elaborately.

Our studies are directed towards a set of heme biomolecules and a few simple iron – porphyrin analogues. We examine the electrochemistry of heme proteins / enzymes and compare them with the electrochemistry of iron – porphyrin complexes. It is mainly the local electronic structure around the active site (primarily the coordination of iron), that controls the redox behaviour of the Fe III/II couple. We try to rationalize changes in the iron redox electrochemistry in terms of local coordination of amino acids to the iron center. With a set of amino acids, we are able to identify a few simple interactions which control redox reversibility of iron in these systems. We use simple electronic structure theory calculations to validate the claims made from electrochemistry data. We believe that an elaboration of the amino acid interaction database to include all known amino acids would per se allow us to predict the electrochemistry of any sequenced heme protein / enzyme, as well as design heme proteins/enzymes de novo with tailored electrochemical properties.

Having understood the nature of the molecular interactions that control redox reversibility of iron in heme systems, we attempt to utilize this in the development of efficient electrochemical sensors for these systems. Confining the molecule on a nanodimensional host on the electrode surface enhances the electrochemical response of the molecule – an ideal situation for use in a biosensor. We investigate this in terms of change in dynamics and active site electronic structure of the protein under confinement. From the point of view of local as well as global dynamics, neutron scattering indicates an ordered diffusion and restriction of peptide backbone fluctuation under confinement. Active site electronic structure, as studied through X ray absorption spectroscopy, indicates minor changes in the local coordination geometry under confinement, presumably due to surface interactions, while long ranges ordering remains unchanged. We use a simple in operando experiment to look at the evolution of electronic structure of the active site with electrochemical cycling.

Overall, we identify molecular interactions around the iron active site in heme systems that control the electrochemical response, attempt to design a confinement based sensor model for heme, and use a host of spectroscopic techniques to identify changes in electronic structure and protein dynamics that make the sensor work efficiently. These studies substantially enrich our understanding of electrochemistry as well as fundamental physical biochemistry of heme enzymes and proteins.