Development of an optimized feeding technology for dairy cows: improvement in resistance to ruminal proteases in the de novo-designed protein MB-1

Resistant Protein: Forms and Functions

Several global health risks are related to our dietary lifestyle. As a consequence of the overconsumption of ultra-processed and highly digestible protein (150–200% of the recommended value), excess dietary proteins reach the colon, are hydrolysed to peptides and amino acids by bacterial proteases and fermented to various potentially toxic end products. A diet reformulation strategy with reduced protein content in food products appears to be the most effective approach. A potential approach to this challenge is to reduce food digestibility by introducing resistant protein into the diet that could positively influence human health and gut microbiome functionality. Resistant protein is a dietary constituent not hydrolysed by digestive enzymes or absorbed in the human small intestine. The chemical conformation and the amino acid composition strictly influence its structural stability and resistance to in vivo proteolysis and denaturation. Responding to the important gap in our knowledge regarding the digestibility performance of alternative proteins, we hypothesise that resistant proteins can beneficially alter food functionality via their role in improving metabolic properties and health benefits in human nutrition, similar to fibres and resistant starches. A multidisciplinary investigation of resistant protein will generate tremendous scientific impact for other interlinked societal, economic, technological and health and wellbeing aspects of human life.

What makes protein indigestible from tissue-related, cellular, and molecular aspects?

This paper gives an insight into key factors, which impair enzymatic protein digestion. By nature, some proteins in raw products are already poorly digestible because of structural peculiarities, or due to their occurrence in plant cytoplasmic organelles or in cell membranes. In plant-based protein, molecular and structural changes can be induced by genetic engineering, even if protein is not a target compound class of the genetic modification. Other proteins only become difficult to digest due to changes that occur during the processing of proteinaceous products, such as extruding, boiling, or acidic or alkaline treatment. The utilization of proteinaceous raw materials in industrial fermentations can also have negative impacts on protein digestibility, when reused as fermentation by-products for animal nutrition, such as brewers' grains. After consumption, protein digestion can be impeded in the intestine by the presence of antinutritional factors, which are ingested together with the food or feedstuff. It is concluded that the encircling matrix, but also molecular, chemical, and structural peculiarities or modifications to amino acids and proteins obstruct protein digestion by common proteolytic enzymes in humans and animals.

The de novo designed nutritive protein MB-1Trp does not resist proteolytic degradation in alfalfa leaves

We previously reported on a de novo designed protein "milk bundle-1Trp" (MB-1Trp) as a source of selected essential amino acids (EAA) for ruminant feeding. Here, we attempt to express this de novo designed protein in alfalfa. The microbial version of the gene encoding the protein was modified in order to achieve two expression strategies in transgenic alfalfa plants. Chimeric MB-1Trp genes alone or fused to a signal peptide and an endoplasmic reticulum retention sequence were introduced into alfalfa via Agrobacterium-mediated transformation. Polymerase chain reaction and reverse transcriptase polymerase chain reaction analysis performed on individual transgenic lines demonstrated that the MB-1Trp gene was correctly integrated and transcribed into mRNA. However, under our conditions, it was impossible to detect MB-1Trp protein expression in any of the transgenic plants analyzed. In order to assess MB-1Trp stability in alfalfa, Escherichia coli-derived MB-1Trp was incubated with proteins extracted from leaves of a non-transgenic plant. This study revealed a high susceptibility of mature MB-1Trp to alfalfa proteases, which may have contributed to its lack of accumulation.

Design of high essential amino acid proteins: two design strategies for improving protease resistance of the nutritious MB-1 protein

Protein design is currently used for the creation of new proteins with desirable traits. In our lab, we focus on the synthesis of proteins with high essential amino acid content having potential applications in animal nutrition. One of the limitations we face in this endeavour is achieving stable proteins despite a highly biased amino acid content. We report here the synthesis and characterisation of two mutants derived from our MB-1 designer protein. The first mutant contains a disulphide bridge designed to cross-link remote segments of the polypeptide chain. The second one is a Tyr62-Trp mutant, where position 62 is buried in the core of the protein. Both mutants were found to be largely helical as per design, and based on thermal denaturation experiments, were substantially more stable than the MB-1 parent molecule. Enhancement of conformational stability in MB-1Trp translated into an impressive improvement of its ability to resist proteolytic degradation. Furthermore, digestion experiments intended to model degradation of proteins in a cow's rumen revealed that MB-1Trp's resistance to degradation compared to that of cytochrome c. Design strategies used for these mutants are discussed with regards to their applicability in creating efficient nutritional proteins.

Engineering nutritious proteins: improvement of stability in the designer protein MB-1 via introduction of disulfide bridges

Protein design is currently used for the creation of new proteins with desirable traits. In this laboratory the focus has been on the synthesis of proteins with high essential amino acid content having potential applications in animal nutrition. One of the limitations faced in this endeavor is achieving stable proteins despite a highly biased amino acid content. Reported here are the synthesis and characterization of two disulfide-bridged mutants derived from the MB-1 designer protein. Both mutants outperformed their parent protein MB-1 with their bridge formed, as shown by circular dichroism, size exclusion chromatography, thermal denaturation, and proteolytic degradation experiments. When the disulfide bridges were cleaved, the mutants' behavior changed: the mutants significantly unfolded, suggesting that the introduction of Cys residues was deleterious to MB-1-folding. In an attempt to compensate for the mutations used, a Tyr62-Trp mutation was performed, leading to an increase in bulk and hydrophobicity in the core. The Trp-containing disulfide-bridged mutants did not behave as well as the original MB-1Trp, suggesting that position 62 might not be adequate for a compensatory mutation.