From NADH Quinone Oxidoreductase (complex I) to Osmolytes : Tale of Two Worlds

Abstract:

NADH quinone oxidoreductase (complex I) is first enzyme in the respiratory chain in mitochondria and many bacteria. It is the largest (~1000kDa) and most complicated enzyme (44-46 dissimilar subunits) in the mitochondrial oxidative phosphorylation system. Many human mitochondrial diseases involve structural and functional defects at the level of this enzyme complex. Bacterial enzyme (NDH-1) is structurally simpler, but functionally similar to the mammalian enzyme. The enzymatic complex has a characteristic L-shaped structure with two distinct domains: a hydrophilic peripheral arm projected into the mitochondrial matrix (or bacterial cytoplasm), and a membrane hydrophobic arm. My research work focused on understanding the structural and molecular mechanism of energy transduction underlying electron transfer and proton pumping in the Escherichia coli NDH-1. Our work provided a solid basis for understanding the electron transfer pathway in the enzyme and revealed missing information regarding the electron flow. Based on our work, we could establish the mechanism of proton-translocation through essential charged residues present in different membrane domain subunits.

Small molecules (osmolytes) are naturally occurring organic compounds. Organisms throughout nature use these compounds against different cellular stresses. Osmolytes have been implicated in both progression and cure of various diseases, including polycystic kidney disease, diabetes mellitus and cancer. They are known to either stabilize or destabilize proteins/nucleotides depending on the concentrations and/or solvent conditions. We developed a high-throughput method for quantifying the energetic impact of addition of various osmolytes on short DNA duplexes. Our result showed that osmolytes had varied effects on DNA stability, and the (de)stabilizing effect does not necessarily correlate with their effects on proteins. The distribution of kidney osmolytes strongly suggests that specific ratios of osmolyte concentrations are needed for proper renal function. By taking a physical chemistry approach we also studied the microscopic solvation properties of the kidney osmolytes in solution. We showed that the six renal osmolytes show quite diverse solvation patterns. Our work thus provides a physical and chemical basis for understanding the importance of osmolyte mixtures in kidney cellular processes.