12 December 2018

Natural protection against nerve gas, drugs and obesity

3D structure of therapeutic enzyme revealed

Structural biologists from Utrecht University found the three-dimensional buildup of HuBChE, an enzyme that protects our nerve system from poisons like pesticides and nerve agents. The results have been published in the scientific journal PNAS yesterday. Group leader Tzviya Zeev-Ben-Mordehai: “Our results will enable production of this therapeutic enzyme in the lab, which has been desired for many years.”

Tzviya Zeev Ben Mordehai near the cryo-EM
Tzviya Zeev-Ben-Mordehai works with a cryo-electron microscope.

First line of defense

“HuBChE, or human butyrylcholinesterase, is a highly abundant enzyme in our blood,” says Tzviya Zeev-Ben-Mordehai, Utrecht University structural biologist and expert in electron microscopy, “It was shown that HuBChE’s primary function is to protect its sister enzyme acetylcholinesterase AChE in short, from damage.” Pesticides and nerve agents from the group of organophosphates are examples of such potentially damaging chemicals. The enzyme AChE enables neuronal transmission in our brains and to our muscles. Damage to this enzyme will lead to paralysis and even death. Zeev-Ben-Mordehai explains how BChE prevents that from happening: “You could say that HuBChE sacrifices itself in case the body gets exposed to organophosphates. It will absorb the deadly chemicals and prevent the harmful substances from reaching AChE in our brain or muscles. BChE is our first line of defense against pesticides and nerve agents.”

HuBChE is a tetramer
HuBChE is a tetramer. Shown as a ribbon model (A) and a molecular surface (B).

Remarkably long circulation time

Tzviya Zeev-Ben-Mordehai and her PhD student Miguel Ricardo Leung now present the three-dimensional structure of HuBChE. Their results were published yesterday by the Proceedings of the National Academy of Sciences of the United States of America (PNAS). “Before our experiments, many of our colleagues had suggested that the enzyme comprises of four identical units. The arrangement of the four subunits is what provides the enzyme its remarkably long circulation time in the blood, which is crucial to its therapeutic power. High-resolution structures of single units were already available. However, we did not know how the four units are arranged relative to each other.”

 

Bouquet of flowers

Zeev-Ben-Mordehai explains how the advanced microscopic technique cryo-EM provided them the new insights. “In cryo-EM, we freeze the protein of interest very rapidly. This preserves its original shape. We keep it frozen during the imaging process. In our research, we found that the four subunits are held together by a central stalk. On a nanoscale, it looks like a bouquet of flowers. We observed that the units are very close but oriented in a way that still allows chemicals to bind to them, giving the enzyme an extraordinary fast enzymatic activity.” The way HuBChE’s four units assemble around the central stalk had not been observed before. “The unique assembly of HuBChE provides the enzyme with its stability in our blood. We suggest to try and mimic this assembly when engineering proteins in order to extend their pharmacokinetic lifetimes.”

HuBChE looks like a bouquet of flowers.
The HuBChE tetramer looks like a bouquet of flowers. Two arrangements of subunits (A: 'open' and B: 'closed') in side view.

Production in the lab

The enzyme HuBChE is already being used as a medicine. It has proved its importance in mitigating the effects of organophosphate nerve agents. Zeev-Ben-Mordehai: “The United States Department of Defense is stockpiling pure human BChE to use for protection in case of nerve gas attacks.” Additionally, BChE suppresses cocaine seeking in rats. This suggests that treatment with BChE could prevent humans from relapsing to cocaine addiction. Furthermore, it was discovered that it also inactivates a hunger hormone, and can thus reduce obesity. “Currently, this important enzyme is collected from expired human plasma, but our results will enable the production of the enzyme in the lab on a large scale” says Zeev-Ben-Mordehai.

 

Publication

Cryo-EM structure of the native butyrylcholinesterase tetramer reveals a dimer of dimers stabilized by a superhelical assembly

Miguel Ricardo Leung*, Laura S. van Bezouwen*, Lawrence M. Schopfer, Joel L. Sussman, Israel Silman, Oksana Lockridge, and Tzviya Zeev-Ben-Mordehai*.

* Affiliated to Utrecht University.

This research is part of the Utrecht University strategic theme Life Sciences.