Lock and key concept

The origin of the remarkable stereoselectivities displayed by chiral homogeneous catalysts has occasioned much interest and speculation. It has been generally assumed, using a lock-and-key concept, that the major product enantiomer arose from a rigid preferred initial binding of the prochiral olefin with the chiral catalyst. Halpren 48) on the basis of considerable evidence, reached the opposite conclusion the predominant product enantiomer arises from the minor, less stable diastereomer of the olefin-catalyst adduct, which frequently does not accumulate in sufficient concentration to be detected. The predominant adduct is in essence a dead-end complex for it hydrogenates at a much slower rate than does the minor adduct. 

HOW EMIL FISCHER WAS LED TO THE LOCK AND KEY CONCEPT FOR ENZYME SPECIFICITY1... 

At this point Fischer concluded that the enzymes, in terms of the configurations of the substrates, are as fastidious as yeast and other organisms. He then returned to the above-mentioned hypothesis that he and Thierfelder had proposed (30) and concluded (32) that the protein substances known as invertin and emulsin, like the substrates whose hydrolyses they effected, were asymmetrically formed molecules. On the basis of this consideration, he came to the momentous lock and key concept for enzyme activity and commented as follows ... 

As already mentioned, the glucoamylase project was chosen to illustrate Emil Fischer s lock and key concept for enzyme specificity. It is seen that his vision has become unequivocally established. Many other developments could have been chosen, as can be appreciated from recent reviews by Hehre (54) and by Svensson (55). Comforth (56) provided a fine overview of asymmetry and enzyme action in his Nobel prize lecture. Noteworthy is the conclusion that stereospecificity is something not just incidental, but essential to enzyme catalysis. In other words, the key must fit the lock. 

Enzymic methods in preparative carbohydrate chemistry, 49, 175-237 Enzyme specificity, how Emil Fischer was led to the lock and key concept for, 50, 1-20... 

Lemieux and Spohr (Alberta) here trace our understanding of enzyme specificity in broad perspective as they assess Emil Fischer s lock and key concept advanced a century ago in relation to current ideas of molecular recognition. It may be noted that the very first article in Volume 1 of Advances, by Claude S. Hudson, was devoted to the Fischer cyanohydrin synthesis and the consequences of asymmetric induction. 

When students learn about enzymes, it is nearly always stated, very dogmatically, that enzymes will act on only one chemical—they have very narrow substrate specificity. Indeed, when introducing the concept of enzyme action at an elementary level, the analogy of the lock and key is often used to illustrate the concept of specificity. In other words, students are told that for every product found in a cell, there will be one enzyme that has made that product and that enzyme will make no other product. This idea was extended to the idea of one gene-one enzyme-one reaction. There is indeed a considerable body of evidence to support the view that many enzymes do have narrow substrate specificity. However, exceptions were known to this rule . But more importantly, the types of enzymes used as good examples of the lock and key concept were drawn... 

Impressed by the specificity of enzymatic action, biochemists early adopted a "lock-and-key" theory which stated that for a reaction to occur the substrate must fit into an active site precisely. Modem experiments have amply verified the idea. A vast amount of kinetic data on families of substrates and related competitive inhibitors support the idea and numerous X-ray structures of enzymes with bound inhibitors or with very slow substrates have given visual evidence of the reality of the lock-and-key concept. Directed mutation of genes of many enzymes of known three-dimensional structure has provided additional proof. 

Figure 25-18 Illustration of the lock-and-key concept of enzyme-substrate Interaction (top) and of the induced-fit theory, whereby the enzyme molds to the substrate through conformational changes (bottom)...

Emil Fischer Berlin Univ., Germany lock-and-key concept (Nobel prize 1902)... 

Important milestones in the rationalization of enzyme catalysis were the lock-and-key concept (Fischer, 1894), Pauling s postulate (1944) and induced fit (Koshland, 1958). Pauling s postulate claims that enzymes derive their catalytic power from transition-state stabilization the postulate can be derived from transition state theory and the idea of a thermodynamic cycle. The Kurz equation, kaJkunat Ks/Kt, is regarded as the mathematical form of Pauling s postulate and states that transition states in the case of successful catalysis must bind much more tightly to the enzyme than ground states. Consequences of the Kurz equation include the concepts of effective concentration for intramolecular reactions, coopera-tivity of numerous interactions between enzyme side chains and substrate molecules, and diffusional control as the upper bound for an enzymatic rate. 

The life and work of one of the greatest carbohydrate scientists of our time, Raymond U. Lemieux, is recalled here in a sensitive account by Bundle (Edmonton). During a remarkably productive career extending over more than half a century, Lemieux pioneered the application of NMR spectroscopy in chemistry, developed rational approaches for glycosidic coupling, made major contributions to our understanding of three-dimensional carbohydrate structures and protein binding, and made important contributions in the biomedical area. His own articles in these Advances include the chemistry of streptomycin in Volume 3, the mechanisms of replacement reactions in Volume 9, and in Volume 50 a consideration of Emil Fischer s lock and key concept of enzyme specificity. 

---