RNA-ийн бүтцийн гол бүрэлдэхүүн хэсэг болох рибоз сахар нь борын эрдэстэй харилцан үйлчилснээр тогтвортой байдлаа хадгалж, эртний Дэлхий дээр оршин тогтносон байж болзошгүйг Австралийн судлаачид Scientific Reports сэтгүүлд нийтэлжээ.
Рибоз нь маш эмзэг бүтэцтэй бөгөөд халуун орчинд хурдан задарч, хэрэгцээгүй “хүрэн зуурмаг” болон хувирдаг шинжтэй. Гэвч борын агууламжтай эрдсүүдтэй нэгдсэнээр энэхүү сахар задралаас сэргийлэгддэг болохыг судалгаагаар илрүүлсэн байна. Bennu зэрэг астероидуудад агуулагдах ийм төрлийн генетикийн бүрэлдэхүүн хэсгүүд нь сансраас Дэлхий рүү тээвэрлэгдэн ирсэн байх магадлалтай гэж эрдэмтэд үздэг.
Шинэ судалгаагаар судлаачид борын агууламжтай кернит, колеманит, улексит зэрэг байгалийн эрдсүүдийг ашиглан рибозтой хэрхэн харилцан үйлчлэлцэхийг туршжээ. Рибоз нь кальциар баялаг чулуулаг дахь борыг уусгаж, өөртэйгөө нэгдэл үүсгэх замаар борын уусах чадварыг 60 хүртэлх хувиар нэмэгдүүлдэг болохыг Энэтхэгийн Пуга халуун рашааны бүсээс авсан дээж дээр хийсэн туршилт харуулав.
Энэхүү үйл явц нь рибоз нь борыг чулуулгаас чөлөөлж, бор нь эргээд рибозыг хамгаалдаг харилцан хамаарал бүхий цикл үүсгэдэг болохыг харуулж байна. Гэсэн хэдий ч судлаачид борын хэт их хэмжээ нь рибоз үүсэхийг саатуулж болзошгүй тул кальци зэрэг бусад бодис нь борын түвшнийг хязгаарлах “тэнцвэржүүлэгч”-ийн үүрэг гүйцэтгэсэн байх магадлалтай гэж таамаглаж байна. Эдгээр олдвор нь амьдралын эх үүсвэр болох RNA-ийн дэлхий дээрх тогтвортой байдлын талаарх шинжлэх ухааны ойлголтыг баяжуулж байна.
Дэлгэрэнгүйг эх сурвалжаас харах
↓Эх сурвалжийг нээх ↓
Ribose, the sugar at the heart of RNA, is notoriously fragile, yet it may have survived on early Earth thanks to a partnership with boron. Chemists and astrobiologists in Australia report that the two substances could have protected each other in a self-reinforcing loop.
Space rocks like Bennu contain key genetic ingredients such as sugar, and they delivered them in explosive fashion, giving early Earth another source of life-friendly materials. Ribose may be one of those arrivals, some of it perhaps showing up from space before terrestrial processes could start making it.
Its importance is hard to overstate. Ribose is the R in RNA, physically forming its backbone, while a modified version gives DNA its skeletal structure. DNA is more famous, but RNA is the one that leaves the nucleus, travelling to the body’s job sites to share genomic data, build proteins, and regulate gene activity.
A Delicate Balance Of Sugar And Chemistry
The trouble is that ribose doesn’t last. Somewhat like table sugar, it turns to caramel under heat and other pressures, though not the delicious kind: a tar-like “brown goop.”
Boron enters the story here. This metalloid forms complexes when it meets ribose, but its availability is fragile too. Calcium and similar agents can pull it out of water and lock it away in rocks, making it useless to burgeoning lifeforms.
Many studies over the past two decades have explored ribose-boron interactions. According to the new paper in Scientific Reports, some of those experiments “may therefore not reflect realistic early Earth environments,” as the researchers put it. So, alongside standard lab-grade borax, the University of New South Wales team used kernite, colemanite, and ulexite, common boron-containing minerals that hold sodium, calcium, or both. They ran each test with and without ribose, the latter acting as a control.
Ribose Pulls Boron Out Of The Rock
Adding ribose changed how boron and calcium interacted. Calcium tends to sequester boron in stubbornly insoluble rocks, but bringing ribose into the mix, a sort of chemical love triangle, offered a solubility solution. Across all four minerals, ribose helped pull boron from its rocky prison into water, and it was especially effective with calcium-containing ones.
The researchers then turned to a piece of “puffy evaporative crust” from the high-altitude Puga hydrothermal system in Ladakh, India, chosen as an early Earth analog. Water alone released a certain amount of boron. With ribose added, the amount released rose by approximately 60 percent.
Degraded ribose was a weaker helper. After the team caramelized it through heat and alkalinity, or chemically, boron solubility rose only slightly. Colemanite’s dissolved boron increased by 50 percent with caramelized ribose, a much smaller boost than fresh ribose gave the same mineral, about 3.5 times more boron. Sucrose and glucose also improved solubility, though not as much as ribose, likely because ribose’s molecular geometry suits it so well to forming stable complexes with boron.
A Loop With A Twist
From these results the researchers propose a loop: ribose pulls and protects boron, which in turn protects ribose. More ribose could mean more dissolved boron, which helps more ribose survive, setting the stage for the RNA World Hypothesis.
But there is a catch. Boron compounds are double-edged swords. They can stabilize existing ribose, yet in excess they inhibit ribose from forming at all. Here calcium goes from villain to hero, since it may have kept boron levels low enough for ribose to form before the loop took over. Shifting pH levels, as may be common in hot spring systems, may also have freed ribose from boron so it could go on to other reactions.
The work also shows the value of natural mineral samples over pure, synthetic ones, even if the latter keep experiments simpler and more controlled. As the paper put it, we come full, or semi circle to Carl Sagan’s idea that we are all star-stuff, specifically, space sugar.
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