Crystal structure of cleaved equine leucocyte elastase inhibitor determined at 1.95 A resolution.

This research article documents the process and result of a team of scientists determining the precise structure of a protein present in horse white blood cells, known as an elastase inhibitor. They used crystallography techniques to achieve a high-resolution image of the protein’s structure after it had undergone a specific cleavage at its active site.

Analysis of the Elastase Inhibitor

• Elastase is a type of enzyme that breaks down proteins. The substance studied in this paper is an inhibitor of elastase, meaning it prevents elastase from doing its job. Specifically, the inhibitor is a serpin. Serpins are a family of proteins that inhibit certain enzymes. In this case, the serpin is inhibiting elastase.
• The researchers studied the elastase inhibitor in horse leucocytes, which are white blood cells. They note that it has unique characteristics, including a low-sequence homology to human alpha 1-antichymotrypsin and alpha 1-proteinase inhibitors. Homology in this context refers to how similar the sequences of amino acids are in the proteins. A low percentage means that the sequences are not very similar.

Structural Findings

• The researchers were able to solve the crystal structure of the active-site cleaved equine leucocyte elastase inhibitor. Solving a crystal structure means defining the exact arrangement of the atoms in the crystal lattice. They reached a crystallographic R-factor of 17.6% at 1.95 A resolution. The R-factor is a measure of the reliability of the structure solution and lower percentages are typically better. The resolution indicates the level of detail the image allows.
• Even though the sequence homology is low, the three-dimensional structures of the elastase inhibitor and the human proteins are very similar. The differences are found in areas where additional segments have been inserted into the structure; these are referred to as “insertions”. There are also differences in several secondary structure elements that have the freedom to move, described as “mobile”.

Significance and Novelty of the Study

• The team managed to achieve a higher resolution compared to that of other studies, enabling them to describe the ‘R-state’ of the serpins more precisely. The R-state refers to the reactive state of the proteins. Being able to detail this further could lead to a deeper understanding of how these types of proteins function and interact with other molecules in the body.
• The findings could potentially have implications for the development of new medical treatments, particularly in understanding the function of similar human proteins. This could be particularly relevant in conditions where the activity of these proteins is key, such as in inflammatory or digestive diseases.