Credit: ELSI

Unlocking Evolution's Secrets with Cell-Free Fe–S Protein Synthesis

Emerging Technologies
September 5, 2023

In a bid to unravel one of evolution's enduring mysteries, Fe–S clusters, the cornerstones of Fe–S proteins present in every life form, have long piqued scientific curiosity. Acting as biological cofactors, these clusters enable intricate biochemical transformations crucial for respiration and metabolism. They offer a captivating blend of pre-biotic chemistry and the evolved molecular systems we witness today, possibly forming the earliest catalysts that ushered in life.

This diagram shows the two main steps involved in the proposed synthesis protocol. Step 1 shows the chemical cascade used to create an oxygen-free environment, alongside the PURE system used to synthesise the 'immature' (apo) proteins. Step 2 shows the implementation of the SUF machinery, which adds the [4Fe–4S] cluster to the apo proteins, yielding functional and mature [4Fe–4S] proteins. (Reproduced from Wang and Nishikawa et al. 2023 ACS Synthetic Biology)

Yet, despite their ubiquity, synthesizing mature Fe–S proteins outside cells has been challenging. Why? Oxygen's touch can degrade their Fe–S clusters. Hence, scientists have been on a twisty path, first deriving an incomplete 'apo' protein and then maturing it, all while steering clear of oxygen. To compound this, unwanted iron-laden proteins often contaminate the end product.

Change, however, is on the horizon. A study led by luminaries, including Associate Professors Kosuke Fujishima and Shawn McGlynn from Tokyo's Earth-Life Science Institute, and Assistant Professor Po-Hsiang Wang, charted a new protocol. Their goal: mature [4Fe-4S] proteins, where the Fe–S cluster forms a geometrically appealing cube. Using a tailor-made Fe–S assembly protein system, they devised a pathway that thrives without oxygen, courtesy of an innovative oxygen-eliminating system. Findings from the new study were published recently in ACS Synthetic Biology.

Delving deeper into the mechanics, the team capitalized on the sulfur formation (SUF) system seen in bacteria. Not only is this multiprotein system a powerhouse in producing [4Fe-4S] clusters, but its resilience to oxygen outperforms other pathways. By creating a recombinant SUF system, they ensured functionality even outside cellular confines.

To keep oxygen at bay, they then implemented a triad of enzymes aptly called the oxygen-scavenging system. As this system purged oxygen, it upped the efficiency ante, generating the vital electron carrier, FADH2, paramount for Fe-S cluster synthesis.

Next, for apo protein creation, the PURE system took center stage. By fusing genetic material with energy sources, it became a potent, artificial protein assembler.

Piecing it all together, the scientists amalgamated the PURE system, oxygen-eaters, and SUF components for a unified synthesis of two flagship [4Fe-4S] proteins. "Achieving the perfect mix of enzymes in the PURE system was arduous," admits PhD student and co-author Shota Nishikawa. Yet their meticulous approach ensured success.

The benefits of this study are manifold. "We've carved a fresh path for Fe–S protein synthesis, negating the need for clunky equipment," beams Fujishima. Their strategy effortlessly sidesteps traditional hurdles, paving the way for cutting-edge biotechnologies and a deeper dive into the enigma of protein formation.

Looking ahead, the team envisions expansive applications. Replicating such oxygen-free realms can birth new synthetic cells and biocatalysts, transforming fields from medicine to astrobiology.

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