Ancient protein model shows early steps in energy metabolism

Researchers in Japan engineered ancestral nucleoside diphosphate kinase variants with restricted amino acid repertoires to explore alternative catalytic functions. One variant, Arc1-12KR, retained its protein fold despite lacking catalytic histidine and unexpectedly converted two ADP molecules into ATP and AMP. The findings reveal alternative catalytic potential in simplified proteins and offer clues to possible routes of energy metabolism during early protein evolution.

Proteins perform essential functions in living organisms, from catalyzing biochemical reactions to supporting cellular energy metabolism. Their functions depend on their amino acid composition and three-dimensional structures. However, early proteins may have evolved when fewer amino acid types were available than those used by most modern proteins. Understanding how proteins built from a more limited set of amino acids could perform essential biochemical reactions may therefore offer insights into the origins and evolution of life.

To investigate this possibility, Professor Satoshi Akanuma and Assistant Professor Sota Yagi of Waseda University, Japan, along with Subrata Dasgupta and Shunsuke Tagami of the RIKEN Center for Integrative Medical Sciences, Japan, examined how restricting amino acid diversity affects the structure and function of ancestral enzymes. Explaining the motivation behind the study, Akanuma said, "We were interested in what happens when an ancestral enzyme is rebuilt using a reduced set of amino acids that may resemble the chemical limitations of early life." The study was published online in Angewandte Chemie International Edition on October 9^th, 2026.

The researchers engineered variants of ancestral nucleoside diphosphate kinase (NDK), an enzyme involved in nucleotide phosphorylation, with progressively restricted amino acid repertoires. They evaluated enzyme activity and reaction products using biochemical assays, high-performance liquid chromatography (HPLC), and mass spectrometry. X-ray crystallography, molecular dynamics simulations, and mutational analyses were used to investigate protein structure, nucleotide binding, and potential catalytic mechanisms.

The findings were interesting. One variant, Arc1-12KR, retained the overall NDK protein fold despite lacking the catalytic histidine typically required for its conventional activity. Unexpectedly, this simplified enzyme converted two molecules of adenosine diphosphate (ADP) into one molecule each of adenosine triphosphate (ATP) and adenosine monophosphate (AMP). HPLC analysis confirmed the formation of both products. Further analyses indicated that aspartate and arginine residues, together with magnesium ions (Mg²⁺), may have helped position the nucleotide molecules and facilitate this alternative phosphoryl-transfer reaction.

Molecular dynamics simulations also indicated that the proposed binding site could accommodate two ADP molecules. However, the precise catalytic mechanism remains unresolved, and alternative pathways cannot be excluded. Arc1-12KR showed an ATP-forming turnover rate of approximately 1.5 s⁻¹. This is about 7,500 times higher than the apparent turnover number reported for short p-loop prototype polypeptides that catalyze the same ADP disproportionation reaction.

Our finding that a simplified ancestral enzyme can form ATP from ADP suggests that early proteins with limited amino acid compositions may have supported primitive energy metabolism."

Satoshi Akanuma, Professor, Waseda University

Although the findings do not establish that this particular enzyme existed on early Earth, they provide an experimental model for exploring how simpler proteins may have supported energy-related chemistry. The findings could also inform the design of artificial enzymes with reduced amino acid repertoires, potentially helping researchers identify new catalytic activities and develop minimal protein catalysts for biotechnology applications.

Overall, the study shows that restricting amino acid diversity can reveal catalytic capabilities beyond a protein's conventional function. By demonstrating that a simplified ancestral enzyme can form ATP from ADP, the research offers a new way to investigate how early proteins may have contributed to the evolution of biological energy metabolism.​​