Physical Chemistry Third Edition

13.1 Catalysis 575

The reaction also exhibitsspecific base catalysisso that the rate law with both

catalyzed mechanisms included is

forward ratek 0 [α]+kH+[H+][α]+kOH−[OH−][α] (13.1-37)

wherekOH−3800 L mol−^1 min−^1 at 18◦C. In basic solution the concentration of

hydrogen ions is small and in acidic solution the concentration of hydroxide ions

is small, so that only one of the last two terms will make a significant contribution

in a given case. However, both terms are still present because introducing another

mechanism does not shut down an existing mechanism.

The reaction also exhibits general acid catalysis. The proposed mechanism is that

either the alpha or beta pyranose ring isomer of glucose is converted to the open-chain

form, which can then close the ring to form either ring isomer:^7

H

HO

O

HA

A



HA 

A

H

HO

O

H

H

HO

O

H

H

HO

O

H

C

H

HO

O

H

C

H

HO

C O

(13.1-38)

where HA stands for a weak acid. The alpha isomer is shown, but the mechanism for

forming the open-chain structure from the beta isomer is analogous, and the mech-

anism for formation of either pyranose ring isomer from the open-chain structure is

the reverse of this mechanism. Step 1 is thought to be rate-limiting, so that the for-

ward reaction is predicted to be first order inα-glucose and first order in the weak

acid HA:

ratek 1 [α][HA] (13.1-39)

Enzyme Catalysis

In cellular biological organisms nearly all chemical reactions are catalyzed by enzymes.

For example, the enzyme urease catalyzes the hydrolysis of urea and the enzyme

protease catalyzes the hydrolysis of proteins. Most enzymes are proteins, although

some ribonucleic acids have been found to exhibit catalytic activity and have been

calledribozymes.^8 Ribozymes usually catalyze the combination of other RNA frag-

ments, and require the presence of divalent metal ions such as Mg^2 +^9. It has been

thought that divalent metal ions were necessary to the function of ribozymes, but

(^7) S. W. Benson,op. cit., p. 558ff (note 6).
(^8) T. R. Cech,Science, 236 , 1532 (1987).
(^9) P. Robertson and W. G. Scott,Science, 315 , 1549 (2007).