Heterogeneous enzyme catalysis

Enzyme catalysis. Enzymes are proteins, polymers of amino acids, which catalyze reactions in living organisms-biochemical and biological reactions. The systems involved may be colloidal-that is, between homogeneous and heterogeneous. Some enzymes are very specific in catalyzing a particular reaction (e.g., the enzyme sucrase catalyzes the inversion of sucrose). Enzyme catalysis is usually molecular catalysis. Since enzyme catalysis is involved in many biochemical reactions, we treat it separately in Chapter 10. 

This survey has been concerned with the enumeration of all possible mechanisms for a complex chemical reaction system based on the assumption of given elementary reaction steps and species. The procedure presented for such identification has been directly applied to a number of examples in the field of heterogeneous catalysis. Application to other areas is clearly indicated. These would include complex homogeneous reaction systems, many of which are characterized by the presence of intermediates acting as catalysts or free radicals. Enzyme catalysis should also be amenable to this approach. 

We need to develop methods to understand trends for complex reactions with many reaction steps. This should preferentially be done by developing models to understand trends, since it will be extremely difficult to perform experiments or DFT calculations for all systems of interest. Many catalysts are not metallic, and we need to develop the concepts that have allowed us to understand and develop models for trends in reactions on transition metal surfaces to other classes of surfaces oxides, carbides, nitrides, and sulfides. It would also be extremely interesting to develop the concepts that would allow us to understand the relationships between heterogeneous catalysis and homogeneous catalysis or enzyme catalysis. Finally, the theoretical methods need further development. The level of accuracy is now so that we can describe some trends in reactivity for transition metals, but a higher accuracy is needed to describe the finer details including possibly catalyst selectivity. The reliable description of some oxides and other insulators may also not be possible unless the theoretical methods to treat exchange and correlation effects are further improved. 

The first reported attempts of what was then called "absolute or total asymmetric synthesis" with chiral solid catalysts used nature (naturally ) both as a model and as a challenge. Hypotheses of the origin of chirality on earth and early ideas on the nature of enzymes strongly influenced this period [15]. Two directions were tried First, chiral solids such as quartz and natural fibres were used as supports for metallic catalysts and second, existing heterogeneous catalysts were modified by the addition of naturally occuring chiral molecules. Both approaches were successful and even if the optical yields were, with few exceptions, very low or not even determined quantitatively the basic feasibility of heterogeneous enantioselective catalysis was established. 

Catalysis by metalloenzymes occupies a relatively unique position when placed in the context of heterogeneous and homogeneous catalysis. For heterogeneous catalysis, it is difficult many times to obtain information on the number and types of atoms at the active site of the catalyst. For enzymic catalysis, a distinguishing feature is that even if one tends to focus on the metal ion as essential to the catalytic center, unique information can be obtained about changes at this one center in spite of the fact that the metal ion is bound to a large protein molecule of molecular weight from 6000 in the simplest cases to over 600,000 in the more complex metalloenzymes. 

The chemistry of the metalloenzymes must be considered as a special case of enzymic catalysis since most active sites of enzymes are stereospecific for only one molecule or class of molecules and many do not involve metal ions in catalysis. Since the metal ion is absolutely essential for catalysis in the examples chosen for this review, the mechanisms undoubtedly involve the metal ion and a particular protein microenvironment or reactive group(s) as joint participants in the catalytic event. It is our belief that studies of catalysis by metalloenzymes will have as many, if not more, features characteristic of protein catalysis in general, in a fashion similar to metal ion catalysis, and these studies will be directly applicable to heterogeneous and homogeneous catalytic chemical systems where the metal ion carries most of the catalytic function. 

Fie. 29. Relationship between homogeneous, heterogeneous, and enzyme catalysis as inferred from the experimental studies of hydrocarbon catalysis on platinum surfaces. 

We may then view the relationship between homogeneous, heterogeneous, and enzyme catalysis as depicted in Fig. 29. The two dominant features of heterogeneous metal catalysis, the importance of low coordination number sites to break chemical bonds and the structural properties of overlayers that control the path of more complex surface reactions, are the bridges between these fields. Future studies will verify how well these views are justified. 

Enzyme catalysis may share some of the characteristics of homogeneous and heterogeneous catalysis, as when the catalyst is a macromolecule small enough to be molecularly dispersed in one phase with all reactants but large enough so that one may speak of active sites on its surface. 

I. Gill and E. Vulfson, Enzymic catalysis in heterogeneous eutectic mixtures of substrates, Trends Biotechnol. 1994, 12, 118-122. 

The mechanism and theory of bioelectrocatalysis is still under development. Electron transfer and variation of potential in the electrodeenzyme-electrolyte system has therefore to be investigated. Whether the enzyme is soluble and the electron transfer process occurs through a mediator, or whether there is direct enzyme immobilization on the electrode surface, the homogeneous process in the enzyme active centre has to be described by the laws of enzyme catalysis, and the heterogeneous processes on the electrode surface by the laws of electrochemical kinetics. Besides this there are other aspects outside electrochemistry or... 

The zeolite is rigid and ordered, and lacks conformational adaptability, in contrast to an enzyme, which can coil, uncoil, and twist around. Yet the zeolite can incorporate transition metal functions—these are of prime importance in enzyme catalysis—and it can effect redox reactions reactions over zeolites can be inhibited by competitive adsorption of reactants, products, solvents, or poisons—a phenomenon observed in biological and some other inorganic heterogeneous catalytic systems Rideal kinetics have been identified in some zeolite-catalyzed alkylations, a pattern which has its parallels in the enzyme field a few cases of stereospecificity (such as orfho-alkylation effects, unusual olefin isomer ratios), where a transition state not otherwise attainable intervenes, may exist. What better group of catalysts than zeolites might there have been to activate the evolutionary process in the dark, fermenting Pre-Cambrian seas some 1,000,000,000 years ago ... 

The initial reaction rate of a catalyzed reaction versus the concentration of the substrate [>q (Eq. (9.39), where K, =k, /ki). The catalytic reaction could be homogeneous, heterogeneous or enzyme catalysis so long as it follows the simple catalytic mechanism. The substrate concentration, [X]. at a tate of half the maximum reaction rate, V, I2, defines in Michaelis-Menten enzyme kinetics. 

With these brief comments, we leave homogeneous and enzyme catalysis to others and concentrate the rest of this book on the development of heterogeneous catalysts. 

Unusual reaction orders are found in product-promoted or reactant-inhibited ("autocatalytic") reactions, the former with positive apparent order with respect to a product, the latter with negative apparent order with respect to a reactant (see Section 8.9). An example of a product-promoted reaction is acid-catalyzed ester hydrolysis. An example of a reactant-inhibited reaction has already been encountered, namely, olefin hydroformylation, whose order with respect to CO is negative (see eqn 6.12 in Section 6.3). Such behavior is also not uncommon in heterogeneous catalysis (see Section 9.3.2) and enzyme catalysis ("substrate-inhibited" reactions in biochemistry lingo, Section 8.3). A reaction having an order with respect to a silent partner—CO in a homogeneous hydrogenation—will be examined in some detail later in this chapter (see Examples 7.3 and 7.4). 

There are three general types of catalysis, depending on the natnre of the rate-increasing snbstance heterogeneous catalysis, homogeneous catalysis, and enzyme catalysis. 

Equation 10.12 is the simplest—and most generally useful—model that reflects heterogeneous catalysis. The active sites S are fixed in number, and the gas-phase molecules of component A compete for them. When the gas-phase concentration of component A is low, most of the sites are empty and the kAd term in the denominator of Equation 10.12 is small so that the reaction is first order in a. When a is large, all the active sites are occupied, and the reaction rate reaches a saturation value oik/kA-The form of Equation 10.12 is widely used for multiphase reactions. The same model, with slightly different physical interpretations, is used for enzyme catalysis and cell growth. See Chapter 12. 

Besides the heavy chemical industry, where catalysis is a dominant feature of most conversion processes, enzyme catalysis is a critical component of bio-chemical processes. All that was said about mechanisms of catalytic reactions applies to enzyme catalysis. As can be expected, there are additional factors in enzyme catalysis that complicate matters. Many enzymatic reactions depend on factors such as pH, ionic strength, co-catalysts and so on that do not normally play a role in conventional heterogeneous catalysis. Despite this, the understanding of mechanisms in enzyme catalysis has outpaced that in heterogeneous catalysis and can now serve as a guide to the search for heterogeneous reaction mechanisms. 

The turnover frequency TOF (the term was borrowed from enzyme catalysis) quantifies the specific activity of a catalytic center for a special reaction under defined reaction conditions by the munber of molecular reactions or catalytic cycles occuring at the center per unit time. For heterogeneous catalysts the number of active centers is derived usually from sorption methods (Eq. 1-7). 

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