An industrial enzyme that survives heat for longer can sound like an immediate improvement. A study of the protein-cutting enzyme PB92 shows why the answer is more complicated: the most heat-resistant variant also had much lower measured activity.
Reporting in Foods in 2023, Huabiao Miao and colleagues investigated ways to improve the thermal stability of PB92, an alkaline protease from Bacillus alcalophilus. Their results show both the possibilities of changing individual amino acids and the trade-offs that must be considered before calling a redesigned enzyme better.
A protein-cutting enzyme under pressure
Proteases cut peptide bonds in proteins. Their practical value depends on more than how fast they work in a standard assay. They also need to remain useful under the temperature, pH and other conditions of the intended process.
The researchers combined structural analysis and sequence comparisons to select positions for mutation. They produced 21 single-substitution variants in Bacillus subtilis and measured activity and heat resistance. Six variants with very low activity were excluded from further characterisation; the remaining 15 had longer measured half-lives at 65°C than the original PB92 under the study’s conditions.
The distinction between activity and stability is important. Activity describes the enzyme preparation’s performance in the assay. Thermal stability describes how well that performance survives exposure to heat. Improving one does not guarantee an improvement in the other.
Longer survival, much less activity
The most striking stability result involved G98E, a substitution of glutamate for glycine at position 98. Its reported thermal half-life at 65°C was approximately 731 minutes, compared with 23 minutes for the original enzyme—about a 32-fold increase.
This half-life was estimated from residual activity after heat treatment. It was not a demonstration that the enzyme continuously processed an industrial feedstock for twelve hours at that temperature.
G98E’s relative extracellular activity was only 6.19% of the original preparation’s activity in the reported comparison. That measurement describes the tested enzyme preparation; it should not be presented as a direct measurement of the catalytic rate of each individual protein molecule.
The result therefore illustrates a substantial trade-off. An enzyme preparation can retain its activity for much longer while starting from a much lower level.
Searching for a better balance
The team also combined substitutions to explore whether useful activity and heat resistance could be retained together. Three combined variants highlighted in the paper had thermal half-lives of roughly 91–105 minutes at 65°C, approximately four to five times the original value.
These were less dramatic stability gains than G98E’s, but the researchers evaluated variants using a combined activity-and-stability score. That approach recognises that the most stable protein is not necessarily the most useful catalyst.
The score was a laboratory screening measure. It did not establish manufacturing yield, operating cost or environmental impact. Which balance is preferable would depend on the actual process, including enzyme dose, reaction time and the material being treated.
Related work on engineering proteins for greater thermal tolerance addresses the same broader challenge: improvements need to be demonstrated for the properties that matter in use.
What the study does—and does not—show
The PB92 experiments provide candidates for further investigation and show how mutations can shift the relationship between activity and heat resistance. They do not demonstrate lower industrial energy use, reduced emissions or a greener production process.
Those outcomes would require tests under realistic operating conditions and a comparison with the existing process. The useful advance here is a clearer view of the engineering choices, including the cost of pursuing the largest stability gain.
Sources
- Miao H. et al. (2023). “Improving the Thermostability of Serine Protease PB92 from Bacillus alcalophilus via Site-Directed Mutagenesis Based on Semi-Rational Design.” Foods, 12, 3081. Original study; open full text.
- PB92 solution structure: PDB 1AH2; structural study.
Update note: This archived article has been revised to explain the activity–stability trade-off and remove unsupported implications about industrial environmental benefits. The original publication date and URL have been retained.
