The enzyme transhydrogenase plays a central role in regulating metabolic processes in animals and humans alike. Malfunction can lead to serious disorders. In every cell, the mitochondria continuously break down molecules derived from food to generate energy as well as to produce new molecules that serve as building blocks of cells.
But by balancing these two opposing processes is accomplishing by an enzyme called proton-translocating transhydrogenase or NNT (nicotinamide nucleotide transhydrogenase). NNT sits in the mitochondria’s membrane and uses the electrochemical proton gradient generated by cellular respiration to provide the mitochondria with just the right amount of the co-enzyme NADPH, a vital metabolic precursor.
High-end microscopy reveals structure and function of crucial metabolic enzyme; The proper functioning of NNT is crucial for metabolic regulation in all animals including humans. However, the details of how NNT accomplishes the coordinated transfer of protons across the membrane and synthesis of NADPH have remained obscure due to the lack of knowledge about the enzyme’s atomic structure.
Regulating metabolic processes
The atomic analysis of the enzyme NNT was only possible by taking advantage of state-of-the-art technology developments in cryo-electron microscopy (cryo-EM); the so-called “resolution revolution.” Parts of the current study’s data were generated using a new cryo-electron microscope installed at IST Austria only in fall 2018.
The cryo-EM analysis of NNT, which involving extensive time- and effort-consuming image processing, delivered near atomic-resolution images of the molecule’s three different domains in various conformational states. Furthermore, membrane proteins like NNT are particularly challenging to study as they are fragile; and difficult to purify in large amounts needing for crystallography.
Proton transfer working
Thus, only with cryo-EM could we finally see clearly how the proton transfer works and with this; find a missing piece of the puzzle on the way to understanding what to do if it does not work. These structures are particularly exciting because transhydrogenase performs an amazing volte-face by rotating an entire; quite large, NADPH-binding domain 180 degrees ‘up’ or ‘down.’ This is, as far as we know; unique among studied enzyme mechanisms.
However, such a rotation now makes complete sense in view of our proposed mechanism; and it shows how nature can ‘creatively’ solve challenging tasks. The results are an important further step toward the development of novel therapies. For instance, the development of currently unavailable NNT inhibitors has great therapeutic potential; with regard to metabolic dysfunctions including metabolic syndrome, and some cancers.