H00102--00A, Front mat Genesis
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206
GENESIS
Autotrophic advocates are equally insistent that true simplicity lies
in building molecules a few atoms at a time, with just a few basic kinds
of chemical reactions. Furthermore, such a mechanism is philosophi-
cally attractive: Autotrophism is deterministic. Rather than depending
on the idiosyncrasies of a local environment for biomolecular compo-
nents, autotrophic organisms make them from scratch the same way
every time, on any viable planet or moon, in a predictable chemical
path. For hardcore advocates of autotrophy, the random, hodgepodge,
hopelessly dilute prebiotic soup is irrelevant to the origin of life. With
autotrophy, biochemistry is hardwired into the universe. The self-made
cell emerges from geochemistry as inevitably as basalt or granite.
BACK TO THE VENTS
As with any metabolic strategy, the Iron–Sulfur World model requires
a source of energy, a source of molecules, and a self-replicating cycle.
Here’s how Wächtershäuser does it.
His model relies on the abundant chemical energy of minerals out
of equilibrium with their environment. He begins by suggesting that
the common iron sulfide mineral pyrrhotite (FeS), which is deposited
in abundance at the mouths of many hydrothermal vents, is unstable
with respect to the surrounding seawater—that is, given time, it will
spontaneously decompose to other more stable chemicals. As a conse-
quence, iron sulfide combines with the volcanic gas hydrogen sulfide
(H S) to produce the shiny mineral pyrite (FeS ) plus hydrogen gas
2
2
(H ) and a jolt of energy:
2
FeS + H S → FeS + H + Energy
2
2
2
Given that energetic boost, hydrogen reacts immediately with the
carbon dioxide (CO ) in seawater to synthesize organic molecules such
2
as formic acid (HCOOH).
Energy + H + CO → HCOOH
2
2
Wächtershäuser envisions cascades of these reactions coupled to
build up essential organic molecules from CO and other simple gases.
2
When these speculations were first proposed, precious little evi-
THE IRON–SULFUR WORLD
207
dence backed them up, but a flurry of experiments in the past decade
and a half has lent support to the Iron–Sulfur World hypothesis.
Wächtershäuser instigated the initial tests, joining with a team of Ger-
man scientists from the University of Regensburg to study the reaction
of iron sulfide and hydrogen sulfide. Sure enough, just as predicted,
they produced pyrite and hydrogen gas. Their brief 1990 paper, “Pyrite
formation linked with hydrogen evolution under anaerobic condi-
tions,” appeared in Nature.
The Iron–Sulfur World hypothesis also makes the unambiguous
prediction that iron sulfide minerals promote a variety of organic re-
actions. In 1996, the Dutch scientists Wolfgang Heinen and Anne Marie
Lauwers simulated Wächtershäuser’s Iron–Sulfur World scenario by
studying reactions of powdered iron sulfide and hydrogen sulfide in
water with an atmosphere of carbon dioxide. As predicted, they found
the synthesis of organic compounds.
Subsequent experiments by Wächtershäuser and colleagues in Ger-
many used iron, nickel, and cobalt sulfides to synthesize acetate, an
essential metabolic molecule with two carbon atoms that plays a cen-
tral role in countless biochemical processes. They expanded on this
success by adding amino acids to their experiments and making pep-
tides—yet another essential step to life.
Our research group in Washington also got into the act. In one set
of gold-capsule experiments with iron sulfide at high temperature and
pressure, we succeeded in producing a variety of organic molecules,
including the key metabolic compound pyruvate. We were especially
interested to learn whether other minerals could perform the same
task. Hydrothermal zones typically feature a variety of minerals in ad-
dition to the common iron sulfides; compounds of nickel, cobalt, zinc,
copper, and other metals abound. So our Carnegie Institution team
ran more than 300 gold-capsule experiments with a dozen different
minerals at high pressure and temperature and found organic synthe-
sis reactions in almost every run.
These experiments have led to an unambiguous conclusion: Com-
mon sulfide minerals can promote a variety of interesting synthesis
reactions. That’s good news for Wächtershäuser’s model, but life re-
quires more than a random assortment of chemical reactions—it
requires a metabolic cycle. What self-replicating cycle of reactions led
to the Iron–Sulfur World?
208
GENESIS
THE REVERSE CITRIC ACID CYCLE
A centerpiece of the Iron–Sulfur World hypothesis is Wächtershäuser’s
conviction that life began with a simple, self-replicating cycle of com-
pounds similar to the one that lies at the heart of every modern cell’s
metabolism—the reverse citric acid cycle. A growing cadre of scientists
concurs, but none has been more outspoken or articulate than Harold
Morowitz, my colleague at George Mason University.
For the better part of two decades, Morowitz has preached a simple
philosophy: If you want to understand the chemical emergence of life,
look to life’s most basic chemistry—the biochemical pathways shared
by all organisms. In the case of metabolism, that core biochemistry is
found in the citric acid cycle, also known to generations of high-school
biology students as the Krebs cycle or the TCA cycle. This circular se-
quence of 11 small molecules, each made of carbon, hydrogen, and
oxygen, forms the core metabolism in every cell. The cycle starts with
the 6-carbon molecule citric acid, which is gradually pared down, a
few atoms at a time, to make a succession of smaller molecules—
5-carbon malate, 4-carbon oxaloacetate, 3-carbon pyruvate, and so
forth—each of which is the starting material for all sorts of other es-
sential biomolecules. Add ammonia to pyruvate and you get the amino
acid alanine; add carbon dioxide to acetate and you get the building
blocks of lipids. Animals and plants use this cycle as the starting point
for synthesizing just about everything biochemical. For many modern
organisms, the citric acid cycle is an engine of biosynthesis.
In the mid-1960s, biologists discovered that some primitive mi-
crobes run the citric acid cycle the “wrong way” round—a pathway
that can be called the “reverse” citric acid cycle. Starting with the 2-
carbon molecule acetate (one of the newsworthy compounds synthe-
sized in Wächtershäuser’s experiments), the cell adds a carbon dioxide
molecule to make 3-carbon pyruvate. Then add another carbon diox-
ide molecule to make 4-carbon oxaloacetate, and so on. Around the
cycle we go, until we get to 6-carbon citric acid. At this point the cycle
does something surprising. Citric acid splits into one acetate molecule
and one oxaloacetate molecule. Suddenly we have the building blocks
for two cycles, so around we go again. The reverse citric acid cycle is a
self-replicating synthesis engine that doubles on every turn. In this way,
a microbe can build all of its essential biomolecules from the simplest
of building blocks—water and gas.
THE IRON–SULFUR WORLD
209
CO2
Acetyl-CoA
Pyruvate
CoA
CO2
Citrate
Oxaloacetate
H2O
H2
Aconitate
Malate
H
H
2O
2O
Isocitrate
Fumarate
H
H
2
CO
2
2
2-oxoglutarate
Succinate
CO2
CoA
Succinyl-CoA
The reverse citric acid cycle is an engine of synthesis that may have been life’s first metabolic cycle.
Harold loves to lecture on the foundations of metabolism, and he
does so with simplicity and elegance. At a recent seminar, he used no
aids but a few hand-scrawled viewgraphs and a big chart of all known
metabolic pathways (he calls it “my security blanket”). At the heart of
that intricate chart lies the simple citric acid cycle—the core chemistry
of life itself. “We’re looking for a fundamental theory of biology,” he
says. “When no one is listening, we call it the Grand Unified Theory of
Biology.”
Harold lectured to the packed, attentive room sitting back in an
easy chair, his arms sweeping in wide circles. He reiterated the basic
concept of the reverse citric acid cycle: Start with two small mol-
ecules—acetate with two carbon atoms and oxaloacetate with four car-
210
GENESIS
bon atoms. Then start tacking on carbon dioxide and water—a carbon
atom here, an oxygen atom there, a few more hydrogen atoms—until,
after just a few steps, you wind up with two citric acid molecules. These
citric acid molecules split to begin new cycles. The various intermedi-
ate molecules of the cycle serve as starting points for the synthesis of all
other key biomolecules, including amino acids, sugars, and lipids. To-
day, cycles of synthesis lie within cycles, which in turn lie within more
cycles, but at the very core is the reverse citric acid cycle. “It’s the way
CO was incorporated into biology before photosynthesis,” explains
2
Harold.
It’s all well and good to point to today’s biochemistry as a clue to
life’s origin, but there’s a key difference between modern cells and the
presumed first metabolic cycle. Today, enzymes—protein catalysts of
incredible complexity—promote each of the 10 separate chemical re-
actions in the “simple” reverse citric acid cycle. One of these catalytic
enzymes speeds up the split of citric acid into acetate and oxaloacetate.
Another facilitates the addition of carbon dioxide to pyruvate to make
oxaloacetate. Without these enzymes to increase chemical efficiency,
today’s cells wouldn’t stand a chance in the competitive struggle for
resources.
How did the first metabolic cycle go it alone, without an enzy-
matic boost? Some of the requisite steps, such as combining carbon
dioxide and pyruvate to make oxaloacetate, are energetically unlikely.
The trouble is that if one step fails, then the whole cycle fails. One of
the clever proposals in Wächtershäuser’s model is that sulfide minerals
promote simple metabolic reactions. It just so happens that many mod-
ern metabolic enzymes have at their core a small cluster of iron or
nickel and sulfur atoms—clusters that look exactly like tiny bits of sul-
fide minerals. Perhaps ancient minerals played the same role as mod-
ern enzymes.
Wächtershäuser invokes another chemical trick based on sulfur. In
his model, the very first cycle was helped along by substituting hydro-
gen sulfide for water (H S for H O) in several crucial reactions, thus
2
2
forming sulfur-bearing analogs of citric acid cycle compounds. He pre-
dicts that this simple chemical substitution leads to faster, energetically
more favorable reactions. What’s more, reactions with sulfide natu-
rally couple to the formation of pyrite—the physical foundation of the
Iron–Sulfur World. Once a sulfide version of the reverse citric acid cycle
was well established on the primitive Earth, it was only a matter of
THE IRON–SULFUR WORLD
211
time before it switched from hydrogen sulfide (which is common only
at volcanic vents) to water (which is everywhere).
EXPERIMENTS
What do experiments have to say about the reverse citric acid cycle? In
the late 1990s, our Carnegie Institution group initiated a series of gold-
tube experiments to see how citric acid degrades in hot, pressurized
water reminiscent of conditions at hydrothermal vents. Ideally, we
would have studied the system with hydrogen sulfide gas, to match the
details of Wächtershäuser’s model, but H S is nasty stuff, technically
2
difficult to load, and potentially deadly if a gas cylinder leaks. We stuck
to the closely related water-based system.
By this time, we had our protocols down pat. I loaded the capsules
with a citric acid solution, Hat Yoder set up the high-pressure runs
(typically at 2,000 atmospheres and 200°C), and George Cody ana-
lyzed the runs. George soon recognized two distinct reaction pathways.
Some citric acid molecules followed what he called the alpha pathway,
which begins with citric acid splitting into acetate plus oxaloacetate,
just like the reverse citric acid cycle is supposed to work. But there the
similarity ended. Oxaloacetate rapidly decomposed to pyruvate plus
carbon dioxide, and much of the pyruvate then decomposed to ac-
etate. Rather than build up new useful molecules, we destroyed the
ones we had.
The next logical step was to take pyruvate and add sulfide minerals
and carbon dioxide—essential ingredients in the Iron–Sulfur World.
But try as we might, no combination of minerals and reactants would
promote the essential reaction from pyruvate, a 3-carbon molecule, to
oxaloacetate, a 4-carbon molecule. Without that step, we had to con-
clude that the alpha pathway is a dead end that destroys citric acid.
Oxaloacetate is the real stopper. Under no plausible prebiotic condi-
tions have we found a way to make oxaloacetate from pyruvate plus
carbon dioxide. Pressure doesn’t seem to help, nor does any realistic
combination of mineral catalysts. Even at modest hydrothermal tem-
peratures less than 100°C, oxaloacetate breaks down rapidly in water.
George didn’t give up. Other citric acid molecules had followed
what he called the beta pathway, which begins in more or less the nor-
mal way, with citric acid giving up a water molecule to make aconitate,
another 6-carbon compound. Then a series of react
ions that release
212
GENESIS
A
B
β′
α
β
George Cody identified two distinct reaction pathways in experiments on citric acid at high temperature and pressure. (A) Some citric acid molecules followed what he
called the alpha pathway, which begins with citric acid splitting into acetate plus oxaloacetate, which then fragments in a nonbiological manner. (B) Other citric acid molecules followed the beta pathway, which begins with citric acid giving up a water molecule to make aconitate, another 6-carbon compound. Then a series of reactions
that release carbon dioxide produced compounds with five, then four, then three
carbon atoms in succession. Cody found evidence that this pathway might be re-
versed to complete a ?? metabolic loop.
carbon dioxide produced compounds with five, then four, then three
carbon atoms in succession.
Is this reaction series another dead end for citric acid? Or might
the beta pathway be part of an alternative self-replicating citric acid
cycle? In subsequent experiments, Cody found evidence that some
steps of the beta pathway can be reversed in the presence of nickel
sulfide, a common hydrothermal vent mineral. Perhaps an abiotic route
from carbon dioxide to citric acid exists after all. We’re still some way
from demonstrating a self-replicating citric acid cycle, but we haven’t
given up yet.
And perhaps someday some brave soul will try these experiments
all over again with hydrogen sulfide—the true test of Wächtershäuser’s
model.
THE IRON–SULFUR WORLD
213
FLAT LIFE
Supposing Wächtershäuser is right, what did the Iron–Sulfur World
look like?
Recall that cellular life requires a membrane—a chemical barrier
to separate the organized insides from the chaotic outsides. Accord-
ingly, most origin-of-life researchers accept the idea of a cell-like lipid
vesicle that surrounded the first self-replicating metabolic cycle. But
an enclosing lipid membrane may not be the only possibility.
Glasgow-based geoscientists Michael Russell and Allan Hall, both
experts in sulfide ore deposits, focus on probable geochemical envi-
ronments in their clever “iron sulphide bubble” scenario. Unlike
Wächtershäuser, they do not rely on the reactive potential of iron sul-
fides to drive metabolism, nor do their sulfides serve as a solid footing
for life. Rather, in their scenario iron sulfide bubbles form spontane-