Rock Eaters ■ 87an orientation that favors the making or breaking
of chemical bonds (Figure 5.8).
Each enzyme binds only to a specific substrate
or substrates and catalyzes a specific chemical
reaction, such as rubisco catalyzing the first step
of the Calvin cycle. An enzyme’s function is based
on its chemical characteristics and the three-
dimensional shape of its active site—the location
within the enzyme where substrates bind. When
molecules bind to the active site, the enzyme
changes shape—a process called induced fit. The
enzyme’s shape, and therefore its activity, can be
affected by temperature, pH, and salt concen-
tration. Because the enzyme’s active site is not
permanently changed as reactions are catalyzed,
an enzyme, like rubisco, is used over and over.
The enzyme that Spormann identified was
unusual because it was excreted to the exte-
rior of the cell. Most enzymes work on the
(^2) Enzyme facilitates
the reaction
among substrate
molecules.
(^4) Enzyme is not permanently
changed by the reaction
and can be recycled.
Induced fit: As molecules
bind to the active site, the
enzyme changes shape to
mold snugly around them.
Substrate molecules
attracted to this
specific enzyme
Catalysis
Enzyme
(^3) Product is
released.
(^1) Molecules bind to the
enzyme at its active site.
Figure 5.8
Enzymes are molecular matchmakers
Enzymes dramatically increase the rate of chemical reactions by positioning molecules so that they
more easily form or break chemical bonds.
Q1: Why is it important that enzymes are not permanently altered when they bind with
substrate molecules?
Q2: How would a higher temperature or higher salt concentration make it more difficult for an
enzyme to function effectively?
Q3: If a cell was unable to produce a particular enzyme necessary for a metabolic pathway,
describe how the absence of that enzyme would affect the cell.
inside of cells. A multistep metabolic pathway
can proceed rapidly and efficiently because the
required enzymes are physically close together
and the products of one enzyme-catalyzed
chemical reaction serve as the basis for the next
reaction in the series, as is especially true for
metabolic pathway reactions.
In the case of M. maripaludis, the excreted
enzyme’s role was to grab an electron from the
metal, pair it with a proton from water, and
create a hydrogen atom—a familiar food for
microbes, and easily passed through the cell
membrane. “They [M. maripaludis bacteria]
found a way to produce a compound that is easily
metabolized by the cells,” says Spormann. And
although this particular microbe wasn’t eating
naked electrons, Spormann has since isolated a
microbe that does directly take up electrons. His
team has yet to publish the details.