The question of whether amino acids in sandwich-like proteins are there to stabilize the structure or to speed up the protein-folding process is best answered by "all of the above," according to researchers at Rice University in Houston.
This discovery, reported in today's issue of the Proceedings of the National Academy of Sciences, could benefit future research on treatments for diseases related to misfolded proteins, such as Alzheimer's and Huntington's.
The Rice scientists studied azurin – a copper-containing protein essential to electron transfer. Azurin is part of a group of proteins that fold into a sandwich-like structure consisting of two sheets of amino acids meshed together. Nearly 70 superfamilies of proteins of varying makeup have this sandwich-like structure, but they all have eight particular amino acids in common. Previous studies had shown that these eight amino acids were important to define the sandwich-like structure, but the exact role was unknown.
"Why are these eight amino acids invariant across all sandwich-like proteins?" asked principal investigator Pernilla Wittung-Stafshede, associate professor of biochemistry and cell biology. "Are they conserved to direct the protein-folding reaction, or are they selected to stabilize the final protein structure? In our paper, we unravel an unprecedented answer to this question."
Wittung-Stafshede and graduate student Corey Wilson analyzed the purpose of six of the eight amino acids by exchanging a nonessential amino acid for each of them and monitoring the effect on the protein structure. (For technical reasons, the other two amino acids could not be studied.) The researchers found that three of the amino acids are important for stabilizing the final structure of the protein, and three serve to direct the process of protein folding.
"We directly demonstrated that in one protein within the large sandwich-like protein family, evolution has indeed preserved amino acids for mechanical reasons," Wittung-Stafshede said. "We believe that our discovery is novel and that it gives important new insight into the interplay between protein evolution, structure and folding."
The researchers speculate that their conclusions about the azurin protein apply to most members of the sandwich-like protein family, but testing on other specific proteins must confirm that.
"Better understanding of protein folding is crucial for curing human diseases directly related to misfolding of proteins, and it is also important for the design and improvement of therapeutic enzymes," Wittung-Stafshede said.
The National Institutes of Health supported the research.
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