October 2001: Photosystem I
Look around. Just about everywhere that you go, you will see
something green. Plants cover the Earth, and their smaller
cousins, algae and photosynthetic bacteria, can be found in
nearly every corner. Everywhere, they are busy converting
carbon dioxide into sugar, creating living organic molecules
out of air using the energy of sunlight as power. This process,
termed photosynthesis, provides the material foundation on
which all life rests.
Capturing Light
At the center of photosynthesis is a
class of proteins termed
photosynthetic reaction centers.
These proteins capture individual
light photons and use them to
provide power for building sugar.
The example shown here is
photosystem I (PDB entry 1jb0), one
of the two large reaction centers
used in cyanobacteria, algae and
plants. Photosystem I is a trimeric
complex that forms a large disk. In
cells, the complex floats in a
membrane (the membrane is
indicated by the two red lines in the
lower picture) with the large flat
faces exposed above and below the
membrane.
Colorful Cofactors
Each of the three subunits of photosystem I is a complex of a
dozen proteins, which together support and position over a
hundred cofactors. Some of these cofactors, shown here in
green and orange, are exposed around the edge of the
complex and many others are buried inside. Cofactors are
small organic molecules that are used to perform chemical
tasks that are beyond the capabilities of pure protein
molecules. The cofactors in photosystem I include many small,
October 2001: Photosystem I
brightly-colored molecules such as chlorophyll, which is bright
green, and carotenoids, which are orange. The colors are, in
fact, the reason that these molecules are useful: the colors are
an indication that the cofactors absorb other colors strongly.
For instance, chlorophyll absorbs blue and red light, leaving
the beautiful greens for us to see. The energy from these
absorbed colors is then captured to perform photosynthesis.
The Electron Transfer Chain
The heart of photosystem I is an electron transfer chain, a
chain of chlorophyll (shown in green), phylloquinone (shown in
orange) and three iron-sulfur clusters (yellow and red at the
top). These cofactors convert the energy from light into energy
that the cell can use. The two chlorophyll molecules at the
bottom capture the light first. When they do, an electron is
excited into a higher energy state. Normally this electron
would quickly decay, releasing heat or releasing a new photon
of slightly lower energy. But before this has a chance to
happen, photosystem I passes this electron on, up the chain of
cofactors. At the top, the electron is transferred to a small
ferredoxin protein (not shown here), which then ferries it on to
the other steps of photosynthesis. At the bottom, the hole left
by this wandering electron is filled by an electron from another
protein, plastocyanin.
October 2001: Photosystem I
This may seem rather mundane until you see the trick that the
photosystem is performing. The proteins at both ends of this
process, ferredoxin and plastocyanin, are carefully chosen.
Because of the special design of their own cofactors, it is more
difficult to add an electron to ferredoxin than it is to
plastocyanin--normally, the flow would be in the opposite
direction. But photosystem I uses the energy from light to
energize the electron, moving it in a difficult direction. Then,
since the electron is placed in such an energetic position, it
can be used to perform unfavorable duties such as the
production of sugar from carbon dioxide.
Photosynthetic Cousins
Different photosystems are used by different photosynthetic
organisms. Higher plants, algae, and some bacteria have the
photosystem I shown here and a second one termed
photosystem II. A low resolution structure of photosystem II is
available in PDB entry 1fe1 (not shown here). Photosystem II
uses water instead of plastocyanin as the donor of electrons to
fill the hole left when the energized electron is passed up the
chain. When it grabs electrons from a water molecule,
photosystem II splits the water and releases oxygen gas. This
reaction is the source of all of the oxygen that we breathe.
Some photosynthetic bacteria contain a smaller photosynthetic
reaction center, such as the one shown on the right (PDB entry
1prc). As in photosystem I, a stack of chlorophyll and other
cofactors transfer a light-energized electron up to an energetic
electron carrier.
October 2001: Photosystem I
Harvesting Light
Of course, plants do not rely on the slim chance of a photon
running into one tiny chlorophyll molecule in the middle of the
reaction center. As with all things in life, cells have found an
even better way. Photosystem I, shown here looking from the
top, contains an electron transfer chain, colored here in bright
colors, at the center of each of the three subunits. Each one is
surrounded by a dense ring of chlorophyll and carotenoid
molecules that act as antennas. In this picture, the protein is
transparent so that only the cofactors are seen. These antenna
molecules each absorb light and transfer energy to their
neighbors. Rapidly, all of the energy funnels into the three
reaction centers, where is captured to create activated
electrons.
October 2001: Photosystem I
Exploring the Structure
You can look at the many photosystem I cofactors of the
electron transfer chain and the antenna in PDB entry 1jb0. Only
one of the three subunits is included in the file, but you will
find that this is complicated enough. If you display only the
cofactors, you will get a picture like the one shown here. This
picture shows the electron transfer chain at the center, drawn
in spacefilling spheres. Two special chlorophyll molecules,
residues 1140 and 1239, are also shown in spheres and
colored green. These two chlorophyll molecules act as a bridge
between the reaction center in the middle and the many
molecules in the surrounding antenna. The many antenna
cofactors are shown here in bond representation with small
spheres for the magnesium ions at the center of each
chlorophyll.
November 2004: Photosystem II
Three billion years ago, our world changed completely. Before then,
life on Earth relied on the limited natural resources found in the
local environment, such as the organic molecules made by
lightning, hot springs, and other geochemical sources. However,
these resources were rapidly being used up. Everything changed
when these tiny cells discovered a way to capture light and use it to
power their internal processes. The discovery of photosynthesis
opened up vast new possibilities for growth and expansion, and life
on the earth boomed. With this new discovery, cells could take
carbon dioxide out of the air and combine it with water to create
the raw materials and energy needed for growth. Today,
photosynthesis is the foundation of life on Earth, providing (with a
few exotic exceptions) the food and energy that keeps every
organism alive.
The Colors of Photosynthesis
Modern cells capture light using photosystem proteins, such as the
one pictured here from PDB entry 1s5l. These photosystems use a
collection of highly-colored molecules to capture light. These lightabsorbing molecules include green chlorophylls, which are
composed of a flat organic molecule surrounding a magnesium ion,
and orange carotenoids, which have a long string of carbon-carbon
double bonds. These molecules absorb light and use it to energize
electrons. The high-energy electrons are then harnessed to power
the cell.
November 2004: Photosystem II
Energetic Electrons
Photosystem II is the first link in the chain of photosynthesis. It
captures photons and uses the energy to extract electrons from
water molecules. These electrons are used in several ways. First,
when the electrons are removed, the water molecule is broken into
oxygen gas, which bubbles away, and hydrogen ions, which are
used to power ATP synthesis. This is the source of all of the oxygen
that we breathe. Second, the electrons are passed down a chain of
electron-carrying proteins, getting an additional boost along the
way from photosystem I. As these electrons flow down the chain,
they are used to pump hydrogen ions across the membrane,
providing even more power for ATP synthesis. Finally, the electrons
are placed on a carrier molecule, NADPH, which delivers them to
enzymes that build sugar from water and carbon dioxide.
The Reaction Center
The heart of photosystem II is the
reaction center, where the energy of
light is converted into the motion of
energized electrons. At the center is
a key chlorophyll molecule. When it
absorbs light, one of its electrons is
promoted to a higher energy. This
energized electron then hops
downward, through several other
pigmented molecules, on to
plastoquinone A, and finally over to
plastoquinone B. When it gets
enough electrons, this small quinone
is released from the photosystem,
and it delivers its electrons to the
next link in the electron-transfer
chain. Of course, this leaves the
original chlorophyll without an
electron. The upper half of the
reaction center has the job of
replacing this electron with a lowenergy electron from water. The
oxygen-evolving center strips an
electron from water and passes it to
a tyrosine amino acid, which then
delivers it to the chlorophyll, making
it ready to absorb another photon.
November 2004: Photosystem II
Harvesting Light
Of course, this whole process wouldn't be very efficient if plants
had to wait for photons to hit that one special chlorophyll in the
reaction center. Fortunately, the energy from a light-excited
electron is easily transferred through the process of resonance
energy transfer. Thanks to the mysteries of quantum mechanics,
the energy can jump from molecule to molecule, as long they are
close enough to each other. To take advantage of this property,
photosystems have large antennas of light-absorbing molecules
that harvest light and transfer their energy inwards to the reaction
center. Plants even build special light-harvesting proteins that sit
next to the photosystems and assist with light collection. The
picture shows a top view of photosystem II (PDB entry 1s5l),
showing all of the light-absorbing molecules inside. The central
chlorophyll molecule of the reaction center is shown with the arrow
(notice the second reaction center in the bottom half--photosystem
II is composed of two identical halves). The little triangular
molecules at top and bottom, stuffed full of chlorophyll and
carotenoids, are light-harvesting proteins (PDB entry 1rwt).
November 2004: Photosystem II
Exploring the Structure
The oxygen-evolving center of photosystem II is a complicated
cluster of manganese ions (magenta), calcium (blue green) and
oxygen atoms (red). It grips two water molecules and removes four
electrons, forming oxygen gas and four hydrogen ions. The actual
binding site of the two water molecules is not known for certain,
but in the PDB structure 1s5l a bicarbonate ion is bound to the
cluster, providing a clue for location of the active site. The picture
shows two oxygen atoms from this ion (colored blue): one is bound
to a manganese ion, the other is bound to the calcium ion. Notice
that the oxygen-evolving center is surrounded by histidines,
aspartates and glutamates, which hold it in place. The tyrosine
shown in the middle forms a perfect bridge between the water site
and the light-capturing chlorophyll.