ATP Synthase, the 'wonder of the molecular world'

In a recent paper published in Nature, researchers at the Institute of Science and Technology Austria, Klosterneuburg, Austria, led by professor Leonid Sazanov, determined the entire structure of F1Fo ATP synthase – the mammalian mitochondrial ATPase that generates ATP. 

"A nano-turbine, a remarkable creation [of nature]," Sazanov called the tiny molecular turbine in an interview with Current Science Daily.

Using cryo-electron microscopy (cryo-EM), the structure of the mammalian ATPase (adenosine triphosphatase) – an enzyme that catalyzes the hydrolysis of phosphates – has been revealed for the first time. Seeing the entirety of the molecular structure, down to the atom, provides insight into how ATP synthase functions to generate ATP. 

The enzyme itself consists of two rotary motors, F1 and Fo, commonly written as F1Fo, which can work separately but, according to Cell, "must be connected together to interconvert energy." 

Until recently, the structure and purpose of F1 was well understood, but not that of Fo. 

"We knew very well how the hydrophilic F1 domain, where ATP is synthesized...looks and works," Sazanov said. 

F1 was mostly mapped out by X-ray crystallography structures completed by professor John Walker of Cambridge, UK. 

"However, the knowledge on structure of the membrane Fo domain, where the flow of protons across the membrane drives the central shaft, was very patchy till now," Sazanov said.  

According to Sazanov, the new high-definition view of the enzyme will allow researchers to fully "visualize and analyze" those proton pathways. "We also identified mammalian-specific features, such as 'hook apparatus' anchoring Fo to the rotating c-ring." 

Sazanov's research also sheds light on the structure of mitochondrial cristae (folds in the inner membrane of mitochondrion that aid in anaerobic cell respiration), as well as the formation of mitochondrial permeability transition pores (PTP) that determine cell death.

How does PTP affect the cell and potentially lead to its death? 

"Permeability Transition Pore (PTP) opening happens in response to high calcium concentrations or due to intense oxidative stress, and leads to mitochondria getting first punctured, then swollen and ruptured, followed by cell death," Sazanov said. 

This phenomenon, according to Sazanov, was discovered 50 years ago. Only recently have researchers begun to find evidence that F1Fo (the tiny molecular motor) might somehow be involved. It remains a controversial topic because other researchers have produced evidence indicating that another pore might contribute to the process, but Sazanov says his team's research shows that F1Fo is directly involved. 

"It was also difficult to imagine previously how exactly F1Fo might play such a role, since calcium binds to F1, far away from the membrane which must harbor PTP," Sazanov said.

Aside from just providing insight into how the F1Fo ATP synthase functions in ATP generation, Sazanov's research also describes how "F1Fo forms dimers and tetramers [that] sit on the highly curved ridges of mitochondrial membranes." The dimers he says may cause this curvature. 

"The shape of inner mitochondrial membranes would be very different without F1Fo, not allowing as tight and efficient packing of proteins," he said.

Sazanov has worked in this field since 1986, which he believes gave him a unique perspective in constructing the current model for how the mitochondrial F1Fo ATP synthase functions. Early in his career, he studied photosynthesis, but eventually moved on to learning mitochondrial and bacterial respiration. His years of research provided the accumulated knowledge and familiarity with "the basic underlying principles behind the operation of proteins and [structures] involved in energy transduction." Energy transduction is the process by which stimuli from the environment are converted into chemical or electrical signals an organism can process. 

ATP synthase represents a corner stone of life. Even a slight deviation in the genome could have severe consequences in the form of debilitating diseases, such as cancer. Sazanov says that the better we understand its structure, the more effective treatments we will be able to develop. 

"We need first to know its atomic structure," he said. "We need to realize that billions of these tiny turbines rotate inside our bodies all the time, generating our weight in ATP per day."