Ea: sample of biochemistry

« In a sense, a squid is the attempt of mollusks to produce a fish. The same basic materials absolve to the same functions, but they do so through completely different structures and systems. So with this planet. Ea is Earth-like in broad strokes – yellow sun, solid surface, molten metal core, liquid water, free oxygen, carbon-based life – but the details, the details are all wrong. » – Eugene Wilson, Darwin Among the Stars

« Complaints that we have lost everything are understandable but wrong. We have all the CHNOPS available. We have sunlight. We have ourselves. That will be enough. Everything else is but details. » – commander Arslana Samirowa, address to Pod 39


Depictions of a number of key macromolecules of Ean biochemistry, i.e. the molecules that make up the organisms of planet Ea. (When a molecule is represented as a jagged line, each angle represents a carbon atom saturated with hydrogen. Element symbols: C, carbon; H, hydrogen; O, oxygen; N, nitrogen; S, sulfur; Zn, zinc.)

a. Paralaminarin. This glucose polymer appears to be Ea's most widespread storage carbohydrate. It resembles the laminarin found in Earth's brown algae, except it has only 2 beta(1→3) links for each beta(1→6) link, rather than 3. It is fully digestible by humans, and has many culinary uses, particularly in Enlilene molecular cuisine. Redstick syrup is up to 30% paralaminarin by dry weight.

b. Pseudoxylan (b1). Another carbohydrate, but mostly structural. It is a polymer of five-carbon xylose (b2), united by beta(1→4) links. Unlike Earth's xylan, its lateral groups are ammonium and carboxyl rather than acetyl and uronic acids. The positively charged ammonium groups and the negatively charged carboxyl groups are believed to be responsible for the ionic bonds that keep pseudoxylan fibers coherent.

c. Zincochrome. The key component of hematophyll, the metalloprotein responsible for photosynthesis in the Hematophytes. The double bonds in the tetrapyrrolic ring allow the absorption of energy from photons. This structure bears an astonishing resemblance to the heme group of hemoglobin (where iron takes the place of zinc), and – if a double bond is broken and the zinc replaced with magnesium – to the chlorin of chlorophyll. The function of the surrounding groups is not yet understood, though they are certainly necessary to anchor the ring into the protein. The copper-containing protein (cyanoglobin) responsible for oxygen absorption in Pentamera probably has a similar structure.

d. Sulfolipid. Ea's equivalent of Earth's phospholipids, sulfolipids are the main components of cell membranes. The apolar portion is formed by two fatty alkyls of various lengths, united to the carbon skeleton by ether links. Ea's triglycerids (common fats and oils) have three fatty alkyl ethers, whereas sulfolipids replace one of them with a negatively charged sulfonate group, which forms the polar portion in contact with the cytoplasm or the external environment. Most xenolytics [i.e. treatments for animals and plants meant specifically to kill Ean pathogens] act by degrading sulfolipids, thereby selectively damaging the cells of Ean organisms.

e. D-amino acids (e1: norvaline/2-aminopentanoic acid; e2: cycloleucine/1-aminocyclopentanecarboxylic acid; e3: homoalanin/alpha-aminobutyric acid). Both the D- and the L-isomers of all these amino acids are found in Terran biochemistry, though the former never form proteins, unlike on Ea. The commonality of amino acids between the two biospheres in not surprising: amino acids were known to exist abundantly in carbonaceous meteorites a full century before the Exodus, as they form spontaneously from methane and ammonia in aqueous solution. Similarly, sugars are produced by relatively simple condensation cycles from formaldehyde.

f. Polysulfonamide (PSA). A structural polymer found in many unrelated Ean organisms. The rigidity of the sulfonyl group causes it to form solid crystals under many conditions. PSA is not as much a molecule as a large class of molecules: he lateral groups (R) can be very different, but they are generally polar and negatively charged. Forms in which R groups are mostly sugars are found in the mucus of many aquatic organisms, whereas forms in which R are long-chain hydrocarbons form the waxy hydrophobic coating of certain cactus-like plants.

g. Thiopolypropylene (TPP). The main component of the exoskeleton of Glyssozoa, as well as the rind of most Dichogastria. Each strand is a chain of syndyotactic polypropylene, that is, one whose lateral methyls regularly protrude in alternate directions. This allows it to form, with sulfur, thioetheric cross-links to the surrounding chains, giving it immense mechanical resistence and a texture that recalls vulcanized rubber. TPP can be produced from transgenic bacterial cultures as a relatively biodegradable form of plastic, since many Ean prokaryotes are capable of digesting it, though rarely at a rate above a few millimeters per year.

h. PNA (peptidonucleic acid) (h1). This molecule carries genetic information thanks to its sequence of nitrogenous bases (e.g. xanthin, h2). It exists in two forms, ketonic (C-C=O, "keto-PNA") and enolic (C=C-O, "enol-PNA"). The first is able to fold on itself and act much like Earth's RNA, performing a number of enzymatic functions, including self-replication. When enzymes convert the keton into enol, the new carbon-carbon double bond makes the backbone much more rigid, and the new negative charge on the oxygen causes them to repel other backbones, forming base-base hydrogen bonds. In the enolic form, PNA forms double helices, like DNA: it's more stable than the ketonic form, but unable to self-replicate. PNA has proven itself a useful substrate for nanocomputation.