The scientists of the institute set out to decipher the secret of the activity of the protease enzymes in the water-repellent lipid environment of the cell membrane
Prof. Deborah Fass and Prof. Eitan Bibi. cut point
change your place change your luck. But does a change in location also change the essence of the action? This question arose when a group of enzymes from a known family was discovered, located in a place where it was not known until now that they might be found. It is a family of protease enzymes, responsible for processes of cutting proteins in the body's cells. These enzymes are common in many animals, and are involved in an even greater number of basic life processes. Due to their wide distribution and their central role, they have been studied widely and in depth. Their structure and mechanisms of activity are known in detail. But recently, evidence has accumulated for the existence of stepsons in the protease enzyme family - which do not work in the cell space, like the other members of the family, but are located inside the cell membrane.
Ostensibly, this is simply a change in the location of the enzyme, but in fact, this location raises a fundamental question. Cutting proteins is a process that requires the presence of water molecules, so it is done in a hydrophilic (water-loving) environment, such as the cell cavity. In contrast, the cell membrane is a lipid environment that repels water. How, then, do the enzymes located in the cell membrane manage to fulfill their purpose and cut proteins? The scientists who set out to find an answer to this question discovered that the protease enzymes located in the cell membrane perform a variety of additional functions: transmitting biochemical communication signals within the cell, controlling programmed cell death, defending against parasite invasion, and more. But not all of these activities are positive. For example, a certain cut performed by these enzymes leads to the formation of segments of the amyloid protein in the cell, the accumulation of which in the brain characterizes Alzheimer's disease.
Prof. Eitan Bibi and the post-doctoral researcher Dr. Adam Ben-Shem from the Department of Biological Chemistry, and Prof. Deborah Fass from the Department of Structural Biology, in the Faculty of Chemistry at the Weizmann Institute of Science, sought to decipher the secret of the activity of the protease enzymes in the water-repellent lipid environment of the cell membrane. To do this, they used an advanced research method, which allowed them to create a detailed, three-dimensional, high-resolution image of a protease enzyme from the E.coli bacterium. The research findings, recently published in the journal "Records of the National Academy of Sciences of the USA" (PNAS), point to a possible solution to the mystery of the mechanism of the enzyme's activity.
The findings show that the enzyme is complex
Six helices cross the cell membrane, and are connected to each other by loops. Five of the helices form a kind of cylinder, as they surround the sixth helix, on which the active site of the enzyme is located. Unlike the five peripheral helices, which cross the membrane across its width and protrude from both sides, the central helix is shorter, so that the active site is immersed in the cell membrane. A pocket-like structure is formed above the active site: its upper part is open to the external environment, and it is bounded on all sides by the protein helices. The inside of the pocket is "lined" with "water-loving" amino acids, and the active site of the enzyme protrudes from the bottom. This special structure allows the penetration of water molecules into the depths of the cell membrane, directly to the place where they are needed.
These discoveries do answer the questions concerning the location of the enzyme's active site, and the way it performs a cut that requires the presence of water, but they raise another question, concerning the protein that the enzyme cuts, which is also located in the cell membrane. How is the access of this protein to the active site, which is in the depths of the enzyme, surrounded by dense protein helices possible? The research findings show several possibilities: one of the rings connecting two of the protein helices may function as a kind of "gate", the opening of which allows the protein to access the active site of the enzyme. Elsewhere, a V-shaped opening was detected between two coils. In any case, since the active site is far from the openings, the researchers hypothesize that a structural change of the enzyme or of the protein intended for truncation is also necessary. "A change in the helical structure of the protein intended for truncation may allow it to access the active site of the enzyme, and also reveal the point of truncation," says Prof. Bibi. "The 'water-loving' environment in the enzyme's active site enables such a structural change - and stabilizes it."
The location of the enzyme, in the cell membrane, makes it difficult for researchers to reveal its additional functions. To do this, the scientists will have to find a way to study it in its natural environment, inside the cell membrane, when it comes into contact with the proteins it cuts. This challenge is at the center of Prof. Bibi's current research.
