Catalytic Activation of Carbon Dioxide - American Chemical Society

Chapter

7

Downloaded by UNIV OF MICHIGAN ANN ARBOR on October 22, 2014 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch007

Enzymatic Activation of Carbon Dioxide Leland C. Allen Department of Chemistry, Princeton University, Princeton, NJ

08544

The kinetics and electronic mechanisms of conventional chemical catalysts are contrasted with those in enzymes. The analogy between certain attributes of surfactants and phase-transfer catalysis and enzyme active sites are made and the limitations of surface catalysts and zeolites are pointed out. The principle features that give enzymes their unusual rate enhancements and remarkable specificity are discussed and ways in which these can be realized in man-made catalysts are proposed. The catalytic activation of CO by both enzymatic and non-enzymatic means, including a detailed analysis of the electronic reaction sequence for the metalloenzyme carbonic anhydrase, is used to illustrate the above themes. 2

In this a r t i c l e we discuss the unique features of enzyme c a t a l y s i s of C 0 hydrolysis and how this may relate to the design of new man-made C 0 catalysts for various reactions of p r a c t i c a l use. The other papers i n this volume are concerned with C 0 c a t a l y s i s at metal and metal oxide surfaces using f i r s t , second, and t h i r d row t r a n s i t i o n metals. Other c a t a l y t i c processes treated i n this volume are electrochemical and photoelectrochemical reduction of C0 . I t i s hoped that further developments of these man-made systems w i l l benefit from a d e t a i l e d analysis of the b i o l o g i c a l c a t a l y s t carbonic anhydrase. This enzyme i s a very o l d one that has evolved to near perfection i n i t s metabolic role of aiding the solvation of C 0 into blood. I t i s one of the simplest enzymes and has one of the highest known turnover rates. We contrast the hydrolysis of C 0 by carbonic anhydrase with that by simple bases to understand the factors contributing to the enormous rate enhancements r e a l i z e d by enzymes, and we describe surfactant and phase-transfer c a t a l y s i s to show how these p a r t i a l l y accomplish some of the benefits of enzymes. We then give the d e t a i l s of the 2

2

2

2

2

2

0097-6156/88/0363-0091 $06.00/0 €> 1988 American Chemical Society

In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

92

CATALYTIC ACTIVATION OF CARBON DIOXIDE

r e c e n t l y d i s c o v e r e d e l e c t r o n i c - l e v e l r e a c t i o n sequence employed by c a r b o n i c anhydrase t o c o n v e r t C 0 t o HC0 ~+H . A l t h o u g h t h i s s p e c i f i c c h e m i c a l r e a c t i o n i s o f l i t t l e c u r r e n t commercial i n t e r e s t , i t i l l u s t r a t e s several general p r i n c i p l e s f o r e f f i c i e n t c a t a l y s i s . A c e n t r a l f e a t u r e o f c a r b o n i c anhydrase and o t h e r enzyme systems i s t h e i r i n h e r e n t t h r e e - d i m e n s i o n a l i t y . P r o p o s a l s f o r u t i l i z i n g t h i s and o t h e r c a r b o n i c anhydrase f e a t u r e s i n t h e s y n t h e s i s o f new c a t a l y s t s a r e d i s c u s s e d . +

2

3


E n z y m a t i c V e r s u s Non-enzymatic C a t a l y s i s o f CO? We c o n s i d e r t h e well-known s i m p l e k i n e t i c model f o r b i m o l e c u l a r r e a c t i o n s between a c a t a l y s t , C, i t s s u b s t r a t e , S, and p r o d u c t , P :

C + S

^cat

CS

C + Ρ

ς

-1 where CS i s t h e c a t a l y s t - s u b s t r a t e complex, an e n t i t y t h a t p l a y s a s i g n i f i c a n t r o l e i n d i f f e r e n t i a t i n g c o n v e n t i o n a l and enzyme c a t a l y s i s . A p p l y i n g t h e s t e a d y s t a t e c o n d i t i o n f o r t h e concen t r a t i o n o f CS l e a d s t o t h e r a t e e x p r e s s i o n : k

c a t

[Co][S]

ν — K

d

+ [S]

where [ C ] i s t h e s p e c i f i e d i n i t i a l c a t a l y s t c o n c e n t r a t i o n , [S] t h e [C][S] k-i cat c o n c e n t r a t i o n , and K = , i s the d i s s o c i a t i o n [CS] k, e q u i l i b r i u m c o n s t a n t f o r t h e CS complex. The e q u a t i o n f o r ν above d e s c r i b e s a r a t e v e r s u s [S] b e h a v i o r t h a t r i s e s l i n e a r l y a t s m a l l [S] and t h e n bends over a t h i g h [S] t o a c o n s t a n t v a l u e w h i c h i s z e r o o r d e r i n [S] . I n most cases o f i n t e r e s t k » k _ and k » k _ , thus K = k a t A l · ( t h a t f o r enzymes t h i s s i m p l e model i s t h e M i c h a e l i s - M e n t e n r e s u l t w i t h K - K , t h e M i c h a e l i s c o n s t a n t , and CS t h e M i c h a e l i s complex). A t low s u b s t r a t e c o n c e n t r a t i o n s t h e r a t e e x p r e s s i o n reduces t o : ν - (k /K )[C ][S] where k / K = k, t h e second o r d e r r a t e c o n s t a n t . For ordinary s o l u t i o n phase c a t a l y s i s , t h e CS a s s o c i a t i o n c o n s t a n t s , 1/K , a r e t y p i c a l l y 10~ -10 « 1 (the l a t t e r value a p p l i e s t o o p p o s i t e l y c h a r g e d i o n s and i s n o t r e l e v a n t t o C 0 as s u b s t r a t e ) . I n c a r b o n i c anhydrase K = 8 x l 0 " M and k 6 x l 0 " » therefore, k = 7.5xl0 M ~ s ~ . Note t h a t t h e CS a s s o c i a t i o n c o n s t a n t i s y i e l d i n g a r a t e enhancement o f « 1 0 . More t y p i c a l l y t h i s f a c t o r i s 1 0 - 1 0 . This i s t h e famous l o c k & k e y n o n - c o v a l e n t b i n d i n g (London d i s p e r s i o n f o r c e s and hydrogen bonds) w h i c h g i v e s r i s e t o an enzyme's v e r y h i g h s u b s t r a t e s p e c i f i c i t y and w h i c h h o l d s t h e s u b s t r a t e i n p r o p e r a l i g n m e n t f o r t h e subsequent e l e c t r o n i c rearrangement o f i t s bonds. I t i s one i m p o r t a n t m a n i f e s t a t i o n o f t h e i n h e r e n t t h r e e d i m e n s i o n a l i t y o f enzyme c a t a l y s i s . S u b s t i t u t i o n o f K = ^cat/^1 Q

+ k

d

c a t

W e n

1

1

d

o

t

d

c a t

c a t

1

e

C

d

M

0

d

d

1

2

2

3

M

1

5 s

c a t

1

7

β

1

2

3

d


4

7. ALLEN

93

Enzymatic Activation of C0

£

i n t o the r a t e equation a t low s u b s t r a t e c o n c e n t r a t i o n g i v e s the second o r d e r r a t e c o n s t a n t as k - k , thus showing t h a t f o r t h i s c o n d i t i o n t h e f o r m a t i o n o f t h e CS complex i s r a t e d e t e r m i n i n g . Second o r d e r r a t e c o n s t a n t s f o r s e v e r a l bases r e a c t i n g w i t h C0 compared t o c a r b o n i c anhydrase a r e shown i n T a b l e I . The 1

2

Table I .

Second Order Rate C o n s t a n t s f o r C 0 R e a c t i o n s i n H 0 a t 25°C

2


2

1

Base a

0H" CH NH C H NH Co(NH ) OH H 0 Carbonic anhydrase

3

8.5xl0 2.0xl0 5.3x10 2.2xl0 6.7xl0" 7 . 5xl0

b

3

4

2

b

5

1

k(M- s- )

5

2

+ a

3

2

5

c

2

d

4

7

a

b

R. B. M a r t i n , J . I n o r g . N u c l . Chem. 38, 511 (1976). M. B. Jensen, A c t a Chem. Scand. 13, 289 (1959). R . G. K h a l i f a h , J . B i o l . Chem. 246, 2561 (1971). G. K h a l i f a h , P r o c . N a t . Acad. S c i . , US, 70, 1986 (1973). C

common f e a t u r e i n a l l o f t h e s e i s t h a t t h e r a t e d e t e r m i n i n g s t e p ( e x c e p t f o r t h e enzyme) i s a t t a c k o f t h e base on t h e c a r b o n o f C 0 and we c a n see t h a t t h e enzyme has a r a t e c o n s t a n t a t l e a s t t h r e e o r d e r s o f magnitude g r e a t e r t h a n any o f them. The bases OH' and C o ( N H ) 0 H (and Z n ( 0 H ) 0 H which i s c l o s e l y a n a l o g o u s ) , as w e l l as c a r b o n i c anhydrase, l e a d t o t h e h y d r o l y s i s o f C 0 and so f u r t h e r comparisons c a n be made. One such comparison i s p K v a l u e s and f o r the t h r e e bases and enzyme t h e s e are : 15.7, 6.6, 6.9, and 7.2 respectively. I f we a r e i n t e r e s t e d i n o p e r a t i o n a t p h y s i o l o g i c a l pH, OH" c a n n o t e x i s t , b u t t h e c o b a l t and z i n c complexes c a n and t h e y t h e r e f o r e (see p a r a g r a p h below) c a n o f f e r a d d i t i o n a l i n s i g h t i n t o t h e unique f e a t u r e s o f enzymatic c a t a l y s i s . A s i m p l e r e a c t i o n sequence s c h e m a t i c f o r t h e u n c a t a l y z e d h y d r o l y s i s ( H 0 as base i n T a b l e I.) i s : 2

+

3

+

5

2

5

2

a

2

ί-

ο

Ο

C - O II

I: S V H C-O II " H . .

c-O

+ HsO+

Ο J T S

Note t h a t t h e r a t e enhancement f o r c a r b o n i c anhydrase o v e r t h e u n c a t a l y z e d r e a c t i o n i s 1.12χ10 , a n o t a t y p i c a l v a l u e f o r many enzymes. F o r t h e z i n c - w a t e r - h y d r o x y l complex a r e a c t i o n sequence schematic i s : 11



94

Ο ί

ο υ

+


ο

Η,0

+

Not u n e x p e c t e d l y , t h e c a r b o n i c anhydrase r e a c t i o n sequence i s more complicated. We g i v e i t and make c o m p a r i s o n w i t h t h e above sequence i n the next s e c t i o n . The r a t e c o n s t a n t k i s that i d e n t i f i e d with the e l e c t r o n i c rearrangement undergone i n t h e r e a c t i o n . From k = k t / K d w i t h K - 1 the k values o f Table I t r a n s l a t e n u m e r i c a l l y i n t o k (s" ) and compared t o c a r b o n i c anhydrase ( k - 6 x l 0 s " ) we see t h a t enhancements range from 30 t o 1 0 . I n a d d i t i o n t o t h e n o n - c o v a l e n t , lock-and-key, e n z y m e - s u b s t r a t e f i t t i n g t h a t c o n s t i t u t e s t h e M i c h a e l i s - M e n t o n CS complex, t h e r e a r e o t h e r s t r u c t u r a l f e a t u r e s o f t h e enzyme-substrate complex w h i c h l e a d t o high k . Thus, an enzyme's a c t i v e s i t e c a v i t y ( f o r c a r b o n i c anhydrase, a cone-shaped i n d e n t a t i o n 12 À i n d i a m e t e r and 12 À deep w i t h a z i n c atom a t t h e bottom) i s l i n e d w i t h a c i d s and bases w h i c h induce t h e e l e c t r o n i c rearrangements t h a t produce t h e c a t a l y s i s . These a r e p o s i t i o n e d t h r e e d i m e n s i o n a l l y such t h a t t h e y have t h e i r maximum c a t a l y t i c e f f e c t when t h e s u b s t r a t e has e l e c t r o n i c a l l y deformed t o i t s t r a n s i t i o n s t a t e c o n f o r m a t i o n . (For carbonic anhydrase t h e amino a c i d s i d e c h a i n s w h i c h c a r r y o u t t h e a c i d - b a s e c a t a l y s i s a r e H i s 64, G l u 1 0 6 ( - ) , Thr 199, a h y d r o x y l a t t a c h e d t o z i n c ( a w a t e r m o l e c u l e d e p r o n a t e d by t h e z i n c i o n ) and a w a t e r m o l e c u l e hydrogen bonded t o T h r 1 9 9 ) . W i t h t h e above i n s i g h t i n t o t h e n a t u r e o f k , we c a n g a i n f u r t h e r knowledge o f enzyme a c t i o n by c o n s i d e r i n g t h e r a t e e x p r e s s i o n a t h i g h s u b s t r a t e c o n c e n t r a t i o n . F o r h i g h [S] a l l t h e c a t a l y t i c s i t e s become f i l l e d w i t h s u b s t r a t e and t h e r a t e i s s o l e l y d e t e r m i n e d by t h e t u r n o v e r r a t e , k . I n t h i s case: c a t

c a

1

d

c a t

5

1

c a t

4

c a t

c a t

c a t

v

k

c

- cat [ ol

We now r e f e r back t o t h e low [S] e x p r e s s i o n where we showed t h e r a t e enhancement advantage o f h a v i n g t h e M i c h a e l i s CS complex and we a l s o n o t e d t h a t k - k , t h e r a t e o f complex f o r m a t i o n . A t low [ S ] , i n c r e a s i n g t h e M i c h a e l i s complex a s s o c i a t i o n c o n s t a n t i n c r e a s e s t h e r a t e * l i n e a r i l y , b u t because o f c a t a l y t i c a c t i v e s i t e s a t u r a t i o n t h i s i s n o t t r u e f o r l a r g e r [ S ] . I f t h e CS b i n d i n g i s too t i g h t , s a t u r a t i o n w i l l o c c u r a t t o o l o w an [ S ] . The CS complex must be j u s t s t r o n g enough (but n o t s t r o n g e r ) t h a n r e q u i r e d t o a c c o m p l i s h i t s purpose o f p r e v e n t i n g an u n n e c e s s a r i l y l a r g e number o f C+S c o l l i s i o n s b e f o r e r e a c t i o n and o f a l i g n i n g t h e s u b s t r a t e f o r i t s subsequent smooth e l e c t r o n i c movement t o t h e t r a n s i t i o n s t a t e . We may now imagine t h e o v e r a l l e n z y m a t i c p r o c e s s b y s t a r t i n g w i t h the u s u a l graph o f ΔΗ v e r s u s r e a c t i o n c o o r d i n a t e w h i c h shows a s i n g l e b a r r i e r between t h e r e a c t a n t and p r o d u c t . Ordinary (non-enzymatic) c a t a l y s i s i s m a n i f e s t b y a s i m p l e l o w e r i n g o f t h i s 1


7.

ALLEN

Enzymatic Activation of

95

CO

i

single b a r r i e r . In enzymes, k values r e f l e c t a greatly lowered main b a r r i e r but there i s also an a d d i t i o n a l small ΔΗ maximum and the Michaelis complex ΔΗ binding minimum which i s introduced between the reactant side of the ΔΗ versus reaction coordinate curve and the main ( k ) t r a n s i t i o n state. The new Michaelis complex binding minimum i n ΔΗ compensates for the large decrease i n entropy that occurs on formation of the Michaelis complex. The new small maximum i n ΔΗ i s the a c t i v a t i o n b a r r i e r to Michaelis complex formation. The various aspects of enzyme action discussed above may be viewed as nature's attempt to create a chemical reaction pathway somewhere between that of a gas phase reaction and one occurring i n pure water solution. The problem of numerous large b a r r i e r s must be overcome i n the gas phase, while i n water, charge separation i s so screened out that large b a r r i e r s can also a r i s e . In the enzyme a l l of these b a r r i e r s are made small and the reaction proceeds along a r e l a t i v e l y smooth middle path. Surfactants and detergents are widely used commercially to provide a favorable environment for numerous organic and inorganic reactions that could otherwise not be c a r r i e d out. Surfactants are made up of hydrophilic and hydrophobic groups which form micelles i n water. The small volume and hydrophobic nature of the micelle i n t e r i o r provides a region that t y p i c a l l y enhances rates by 10 . A p a r a l l e l event i s happening i n enzymes and i s part of the e f f e c t we have ascribed to Michaelis complex binding and entropy reduction. The enzyme active s i t e excludes almost a l l water molecules (except those which are an i n t e g r a l part of the reaction mechanism or those that act as s t r u c t u r a l elements i n o r i e n t i n g substrate for chemical reaction). Exclusion of water provides a d i e l e c t r i c constant of near unity and allows easy separation of charges - the key feature required to lower the energy for charge rearrangements i n the t r a n s i t i o n state. Phase-transfer c a t a l y s i s i s another much employed i n d u s t r i a l process that also bears some analogy to enzyme active s i t e s . Again, the advantage of carrying out ionic reactions i n a low d i e l e c t r i c i s r e a l i z e d i n the presence of two immissible solvent media (generally water and a hydrocarbon). c a t


c a t

3

The E l e c t r o n i c Reaction Mechanism of Carbonic Anhydrase In spite of the fact that enzymology i s an old and well-established d i s c i p l i n e , e l e c t r o n i c - l e v e l understanding of enzyme mechanisms (such as we have for a number of organic and inorganic reactions) i s only now j u s t beginning to emerge for one or two enzymes. Carbonic anhydrase i s one of these. Likewise, although there are numerous textbooks which put forth mechanistic hypotheses, none of them o f f e r long-standing, u n i v e r s a l l y agreed-upon enzyme reaction sequence schematics at the e l e c t r o n i c l e v e l and, at present, one must r e l y on the current l i t e r a t u r e . For carbonic anhydrase, a great deal of e f f o r t has been expended over many years i n many laboratories, and the work on i t s mechanism has recently been reviewed (1-2). The computational techniques for constructing the reaction p o t e n t i a l energy surfaces and t h e i r r e l a t i o n to experimental measurements has also been treated (2-4). For a long time there was a c o n f l i c t between spectroscopists - p a r t i c u l a r i l y those employing NMR - and


96


k i n e t i c i s t s as to the role of water molecules i n the reaction, but a systematic computational testing of the various mechanistic hypotheses appears to have l e d to a concensus among the experimentalists. We focus our attention here on that part of the reaction which converts C0 into HC0 " and i s o l a t e those electronic features which might be most e a s i l y simulated i n man-made catalysts. (The other part of the mechanism involves buffer and the movement of a proton i n and out of the active s i t e and i t i s unique to carbonic anhydrase's physiological role i n metabolism). Figure 1 shows the arrangement of amino acid residues i n the carbonic anhydrase active s i t e and Figure 2 reduces t h i s to a two-dimensional working schematic. I n i t i a l l y the active s i t e contains a zinc-bound hydroxyl, and a water molecule bound d i s t a n t l y as a zinc f i f t h ligand. This water molecule i s hydrogen bonded to Thr 199 which i n turn i s hydrogen bonded to Glu 106(-). Glu 106(-) i s exposed to solvent, i s r e l a t i v e l y near zinc (2+) and i s hydrogen bonded to the backbone nitrogen of Arg 246. A l l of these bonds s t a b i l i z e the negative charge on Glu 106(-). The zinc-bound hydroxyl i s hydrogen bonded through a water chain to His 64. In the absence of the C0 substrate an additional water molecule w i l l be hydrogen bonded to the hydroxyl and this must be displaced by the incoming C0 . Figure 3 shows the detailed reaction sequence. Incoming C0 w i l l be attacked by the zinc-bound hydroxyl (B) forming a c y c l i c intermediate with the f i f t h coordinate water. Additional hydrogen bonds with one or more water molecules i n the active s i t e l i n k the bicarbonate proton to the exocarboxylate oxygen. As the c y c l i c intermediate i s formed a unit of negative charge moves from the hydroxyl to bicarbonate. This w i l l be compensated by a p a r t i a l proton d r i f t from the f i f t h coordinate water through Thr 199 to Glu 106. The proton a f f i n i t y of the hydroxide w i l l also drop because of C-O formation, thereby allowing a proton transfer to the exocarboxylate oxygen (C). His 64 s t a b i l i z e s the t r a n s i t i o n state for t h i s transfer. The loss of the bicarbonate product from the active s i t e passes through the t r a n s i t i o n structure (D). As the zinc oxygen bond breaks, a negative charge (shown as « h) w i l l develop on the oxygen. A proton from the zinc-bound water transfers through Thr 199 to Glu 106 regenerating the zinc-bound hydroxyl. This "around the corner SJJ2 reaction" leads to a lowered binding energy for the bicarbonate product and thus to i t s release. In this process zinc merely trades anionic ligands rather than having to break a f u l l ionic bond. The reaction cycle i s completed i n two steps: the addition of a water molecule and loss of a proton to solvent. The reaction sequence shown i n Figure 3 uses two water molecules as an intimate and rather subtle part of the mechanism and i t i s well to bring out how this occurs. The f i r s t water i s the lower one i n the diagram: i t gets deprotonated by Z n and the r e s u l t i n g hydroxyl acts as the p r i n c i p l e attacking nucleophile i n the reaction. This molecule i s further deprotonated i n the proton transfer to the exocarboxylate oxygen and therefore a strong Zn-0 bond i s established. This Zn-0 bond must i n turn be broken to release product. The electronic r e d i s t r i b u t i o n which accomplishes t h i s i s the around the corner Sjj2 reaction' - one of nature's clever inventions that mankind w i l l want to copy. The lower water


2

3

2

2

2

2 +

N



7. ALLEN

Enzymatic Activation of CO

Figure 1.

g

Perspective view of carbonic anhydrase active s i t e . The arrow points toward the opening of the cavity.

k

N

Ù

Figure 2 .

ARG 246

Two dimensional schematic of carbonic anhydrase active s i t e and C 0 substrate showing those parts of the enzyme s p e c i f i c a l l y involved i n catalysis. 2


97


Η

,Η Η

Η

V /Η Ο-Η-0

Figure 3.

2

+co

h

2

Im--o'

ί

0"Η-ρ'

Η

Electronic reaction sequence f o r the c a t a l y t i c conversion of C0 to HCO3 by carbonic anhydrase.

Β

\/V

VH-9'

H-cf-

Λ,. Im--o'



7.


ALLEN

99

CO

g

also contains the ^oxygen of hydrolysis' - that oxygen which becomes incorporated into the product - and as such isotopic labeling readily identifies i t . The second, upper, water acts as part of a 'working medium': i t makes possible a c y c l i c t r a n s i t i o n state and i t s upper hydrogen moves o f f and on to i t s oxygen i n response to the r e l a t i v e proton a f f i n i t y of the forming c y c l i c t r a n s i t i o n state versus that of the Glu 106 - Thr 199 change relay. Zn-0 f o r t h i s water starts out as a long bond, but becomes a short bond as i t passes through the t r a n s i t i o n state of (D). When this bond becomes short i t allows the lower Zn-0 bond to become long and break. C a t a l y t i c P r i n c i p l e s and Design of More Powerful Catalysts +

It i s now apparent why Z n ( 0 H ) ( 0 H ) i n water s o l u t i o n i s a r e l a t i v e l y i n e f f i c i e n t c a t a l y s t compared to carbonic anhydrase: although the 0····Ζη bond i n the t r a n s i t i o n state of the reaction schematic given i n the section above i s shown as a dotted l i n e , there i s no e l e c t r o n i c rearrangement path available to a i d i n breaking this bond (the TS could also have been written i n a c y c l i c form - Z 0C0Z - but the same problem a r i s e s ) . The ^around the corner SJJ2 reaction' solves this problem. A c l o s e l y related feature i s the p r a c t i c a l l y constant charge of « + 1 maintained on zinc throughout the entire reaction sequence (which i s another way of saying that the a c t i v a t i o n b a r r i e r s f o r the e l e c t r o n i c rearrangements i n going from reactant to product are small). These observations are generalized by the p r i n c i p l e (5) of "two bond maneuver' (Figure 4) which exists i n a l l enzymes we have studied. I f one bond i s broken and one new bond made i n a c a t a l y t i c process then i t i s necessary to supply enough energy to break that bond even though a nearly equal amount i s returned when another bond i s made. This can be avoided by simultaneously breaking and making two bonds (one i n the enzyme and one i n the substrate) because each atom maintains a roughly constant charge around i t s e l f . Moreover, we can see from the atomic charge assignments i n Figure 4 that bond p o l a r i t i e s are also maintained and this implies the opportunity for low ΔΗ b a r r i e r s . What are the e s s e n t i a l chemical properties provided by zinc and i t s ligands i n carbonic anhydrase? ( I t may be noted that replacement of zinc by cobalt and cadmium has been c a r r i e d out and for the cobalt s u b s t i t u t i o n there i s no change i n properties while 2

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The 'Two Bond Maneuver' i n enzymes (D. E. Green, Proc. Nat. Acad. S c i , U.S. 78, 5344 (1981)) whereby a c t i v a t i o n b a r r i e r s are made low by the simultaneous breaking and making of two bonds i n the enzyme and substrate.


100


the cadmium enzyme s t i l l functions but at a low l e v e l ) . Zinc i s acting as a strong Lewis acid ( e f f e c t i v e charge +1) and i n addition this atom needs to be of s u f f i c i e n t size and p o l a r i z a b i l i t y to maintain a roughly constant charge without large energy changes. This r e l a t i v e l y simple f u n c t i o n a l i t y i s the central role for zinc i n a l l i t s enzymes and the same statement can be made for Mg and for about h a l f of the enzymes u t i l i z i n g Mn, Fe, Co, and Cu. Also because of these simple requirements, i t i s not surprising to f i n d that nature has evolved two classes of enzymes that carry out the same chemical reaction by the same general mechanism, one of which u t i l i z e s zinc and the other N H ( i n the form of a S c h i f f base). These are the fructose diphosphate aldolase enzymes i n the g l y c o l y t i c pathway which cleave a C-C bond and leave a C=0 on one of the products. Another c a t a l y t i c p r i n c i p l e can be learned from carbonic anhydrase and two other zinc enzymes which have been wellcharacterized: carboxypeptidase A and l i v e r alcohol dehydrogenase. In each of these, Zn i s held into i t s enzyme by three chemically i n e r t ligands, but these ligands d i f f e r i n t h e i r charge so that the formal charge on Zn i n these three cases i s 2+, 1+, and 0 respectively. Zinc i s acting as a successively weaker Lewis acid, but the e l e c t r o n i c rearrangements going on around zinc during c a t a l y s i s have a remarkable s i m i l a r i t y . In each case the zinc i s 5-coordinate, 3 from the inactive ligands and 2 from two oxygen atoms that are a c t i v e l y p a r t i c i p a t i n g i n electron flows. One of the oxygen atoms i s attached to the substrate and the other to a water molecule. (This i s the upper water i n the carbonic anhydrase mechanism which changes i t s Zn-0 distance and i t s oxygen charge during the reaction). In e f f e c t , i n each case, the c a t a l y s t i s zinc plus the water molecule, with the f i f t h coordination s i t e u t i l i z e d by the substrate. The concept of a mobile s a t e l i t e molecule containing an electronegative atom that acts as an electron switching agent may be applicable to other s i t u a t i o n s . There are two other general themes of enzyme c a t a l y s i s i l l u s t r a t e d by carbonic anhydrase that bear d i r e c t l y on contemporary commercial c a t a l y s t s . The f i r s t i s the inherent three dimensionality that we have noted several times. Almost a l l of the many, many v a r i e t i e s of i n d u s t r i a l catalysts u t i l i z e the properties of a two-dimensional surface, with the c a t a l y t i c event t y p i c a l l y taking place at the p o t e n t i a l energy d i s c o n t i n u i t i e s created by steps' or islands' on the surface. Using only two dimensions i s a very severe l i m i t a t i o n because one cannot control the environment surrounding the atoms. One needs to use a l l three dimensions f u l l y to do t h i s . This does not mean that excellent catalysts can't be developed, but i t does mean that t h e i r design w i l l be governed by pure t r i a l and error to a very large extent and suggests that we are far less advanced i n our technological c a p a b i l i t i e s than we might be. The second theme comes from examining the properties of z e o l i t e s . Here, the three dimensional nature of these catalysts has long been exploited by the petroleum industry, mostly because they function as molecular sieves. Z e o l i t e s , l i k e a l l of the mineral s i l i c a t e s of which they are a s p e c i a l c l a s s , are constructed with highly ionic bonds and this introduces two severe l i m i t a t i o n s . F i r s t , the number of possible structures made out of ionic bonds i s much less diverse than those possible from covalent bonding, and


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second, the large p o s i t i v e and negative charges d i s t r i b u t e d i n more or less fixed positions throughout the z e o l i t e s i s not conducive to r e a l i z a t i o n of the subtle charge redistributions and b a r r i e r lowerings required for r e a l l y f l e x i b l e and v e r s a t i l e catalysts. From the above i t seems clear that i n order to b u i l d f u l l y controllable three dimensional c a v i t i e s l i n e d with appropriate functional groups i t i s probably necessary to follow nature's lead and use an organic polymer made up of units with common end groups to permit easy construction of a chain. However, the polypeptide chains of proteins have their own problems: i t should not be necessary to generate α-helices and β-pleated sheets i n order to get stable and reasonably compact structures. Therefore, simpler polymer building blocks, without the p o s s i b i l i t y of forming hydrogen bonds, might be useful. Likewise, one probably doesn't need a l l twenty of the amino acid side chains used i n proteins and the functional groups employed are not ideal for many of the kinds of man-made molecules that are desired. Thus, a smaller and simpler set might be used for attachment to the organic polymer building-block units. Experimentation with a polymer such as described above would s t a r t by using a very few links to explore what types of configurations could be realized. This exploration would be greatly aided by the powerful molecular mechanics computer programs that have been under intensive development during the l a s t few years. I t i s not at a l l unreasonable to expect that conformations for at least the simple polymers can be predicted accurately. The combination of modern computational methods along with the continuing strong advances i n polymer chemistry w i l l match the incredible accomplishment of nature's evolution more quickly than could at f i r s t be imagined. These man-made analogies to enzymes w i l l play a key role i n the nanotechnology revolution (the development of molecular machines and devices at the 10 À level) predicted a few years ago by Profs. Richard P. Feynman and Freeman Dyson. References to the a r t i c l e s by Feynman, Dyson and several other seminal thinkers i n this area are given i n a recent popular account by Κ. E. Drexler (6). Acknowledgments The author wishes to thank the Office of Naval Research and the National Institute of Health for f i n a n c i a l support of this research.

Literature Cited 1. Cook, C. M.; Lee, R. H.; and Allen, L. C. Inter. J. Quant. Chem. Sym. 1983, 10, 263. 2. Cook, C. M; and Allen, L. C. Biology and Chemistry of Carbonic Anhydrase, Tashian R. E. and Hewett-Emmet, D., Eds., Ann. N.Y. Acad. Sci. 1984, 429, 84. 3. Cook, C. M.; Haydock, K.; Lee, R. H.; and Allen, L. C. J. Phys. Chem. 1984, 88, 4875. 4. Allen, L. C. Ann. Ν. Y. Acad. Sci. 1981, 367, 383. 5. Green, D. E. Proc. Nat. Acad. Sci. U.S. 1981, 78, 5344. 6. Drexler, Κ. E. Engines of Creation; Anchor Press/Double Day Ca., 1986. RECEIVED December 1, 1986


Catalytic Activation of Carbon Dioxide - American Chemical Society

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