The Foundations of Chemistry

Enzyme-catalyzed reactions are important examples of zero-order reactions; that is, the
rate of such a reaction is independent of the concentration of the substrate (provided some
substrate is present).

ratek

The active site on an enzyme can bind to only one substrate molecule at a time (or one
pair, if the reaction links two reactant molecules), no matter how many other substrate
molecules are available in the vicinity.
Ammonia is a very important industrial chemical that is used as a fertilizer and in the
manufacture of many other chemicals. The reaction of nitrogen with hydrogen is a ther-
modynamically spontaneous reaction (product-favored), but without a catalyst it is very
slow, even at high temperatures. The Haber process for its preparation involves the use
of iron as a catalyst at 450°C to 500°C and high pressures.

Fe

N 2 (g)
3H 2 (g)888n2NH 3 (g) G^0 194.7 kJ/mol (at 500°C)

Even so, iron is not a very effective catalyst.
In contrast, the reaction between N 2 and H 2 to form NH 3 is catalyzed at room temper-
ature and atmospheric pressure by a class of enzymes, called nitrogenases, that are present
in some bacteria. Legumes are plants that support these bacteria; they are able to obtain
nitrogen as N 2 from the atmosphere and convert it to ammonia.
In comparison with manufactured catalysts, most enzymes are tremendously efficient
under very mild conditions. If chemists and biochemists could develop catalysts with a
small fraction of the efficiency of enzymes, such catalysts could be a great boon to the
world’s health and economy. One of the most active areas of current chemical research
involves attempts to discover or synthesize catalysts that can mimic the efficiency of natu-
rally occurring enzymes such as nitrogenases. Such a development would be important in
industry. It would eliminate the costs of the high temperature and high pressure that are
necessary in the Haber process. This could decrease the cost of food grown with the aid
of ammonia-based fertilizers. Ultimately this would help greatly to feed the world’s
growing population.

Transition metal ions are present in

the active sites of some enzymes.

Enzyme bound

to substrate

Substrate

cleaved

Enzyme

unbound

Substrate Enzyme

Figure 16-19 A schematic representation of a simplified mechanism (lock-and-key) for
enzyme reaction. The substrates (reactants) fit the active sites of the enzyme molecule much
as keys fit locks. When the reaction is complete, the products do not fit the active sites as
well as the reactants did. They separate from the enzyme, leaving it free to catalyze the
reaction of additional reactant molecules. The enzyme is not permanently changed by the
process. The illustration here is for a process in which a complex reactant molecule is split
to form two simpler product molecules. The formation of simple sugars from complex
carbohydrates is a similar reaction. Some enzymes catalyze the combination of simple
molecules to form more complex ones.

The process is called nitrogen fixation.

The ammonia can be used in the

synthesis of many nitrogen-containing

biological compounds such as proteins

and nucleic acids.