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Enzyme

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An Introduction toEnzymes

Much of the history of biochemistry is the historyof enzyme research. Biological catalysis was first

recognized and described in the late 1700s, in studies

on the digestion of meat by secretions of the stomach,

and research continued in the 1800s with examinations

of the conversion of starch to sugar by saliva and

various plant extracts. In the 1850s, Louis Pasteur

concluded that fermentation of sugar into alcohol by

yeast is catalyzed by ferments.

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He postulated that these ferments were inseparable

from the structure of living yeast cells; this view, called

vitalism(), prevailed for decades. Then in 1897Eduard Buchner discovered that yeast extracts could

ferment sugar to alcohol, proving that fermentation waspromoted by molecules that continued to function when

removed from cells. Frederick W. Khne called these

molecules enzymes. As vitalistic notions of life were

disproved, the isolation of new enzymes and the

investigation of their properties advanced the science of

biochemistry.

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The isolation and crystallization of urease byJames Sumner in 1926 provided a breakthrough in

early enzyme studies. Sumner found that urease

crystals consisted entirely of protein, and he

postulated that all enzymes are proteins. In the

absence of other examples, this idea remained

controversial for some time. Only in the 1930s was

Sumners conclusion widely accepted, after JohnNorthrop and Moses Kunitz crystallized pepsin,

trypsin, and other digestive enzymes and found

them also to be proteins.

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During this period, J. B. S. Haldane wrote a

treatise entitled Enzymes. Although the molecular

nature of enzymes was not yet fully appreciated,

Haldane made the remarkable suggestion that

weak bonding interactions between an enzyme and

its substrate might be used to catalyze a reaction.

This insight lies at the heart of our current

understanding of enzymatic catalysis.

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With the exception of a small group ofcatalytic RNA molecules, all enzymes are

proteins. Their catalytic activity depends on the

integrity of their native protein conformation. If anenzyme is denatured or dissociated into its

subunits, catalytic activity is usually lost. If an

enzyme is broken down into its component aminoacids, its catalytic activity is always destroyed.

Thus the primary, secondary, tertiary, and

quaternary structures of protein enzymes are

essential to their catal tic activit .

Most Enzymes Are Proteins

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pH

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The sweet taste of freshly picked corn (maize) is

due to the high level of sugar in the kernels. Store-

bought corn (several days after picking) is not as

sweet, because about 50% of the free sugar is

converted to starch within one day of picking. To

preserve the sweetness of fresh corn, the husked

ears can be immersed in boiling water for a few

minutes (blanched) then cooled in cold water. Corn

processed in this way and stored in a freezer

maintains its sweetness. What is the biochemical

basis for this procedure?

question?-- Keeping the Sweet Taste of Corn

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substrateproduct

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1.

;;

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105~1017

2.

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()

2 H2O2 2 H2O + O2

1000 109

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2.

H+

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2.

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specificity)

1.

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2N NH2C

O

2NH3+ CO2

H2O

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2 A group specificity)A-B

OCH2OH

OHOH

OH

O R

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2

B bond specificity)A-BA,B

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C stereo specificity) a. :

L + NH3+ H2O

L

H2O + O2

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C stereo specificity) B. :HOOC CH

HC COOH

CH2COOH

CHCOOHHO

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Summry

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