Enzyme lock-and-key model

Living cells contain thousands of different kinds of catalysts, each of which is necessary to life. Many of these catalysts are proteins called enzymes, large molecules with a slotlike active site, where reaction takes place (Fig. 13.39). The substrate, the molecule on which the enzyme acts, fits into the slot as a key fits into a lock (Fig. 13.40). However, unlike an ordinary lock, a protein molecule distorts slightly as the substrate molecule approaches, and its ability to undergo the correct distortion also determines whether the key will fit. This refinement of the original lock-and-key model is known as the induced-fit mechanism of enzyme action. 

FIGURE 13.40 In the lock-and-key model of enzyme action, the correct substrate is recognized by its ability to fit into the active site like a key into a lock. In a refinement of this model, the enzyme changes its shape slightly as the key enters. 

Early in the last century, Emil Fischer compared the highly specific fit between enzymes and their substrates to that of a lock and its key. While the lock and key model accounted for the exquisite specificity of enzyme-substrate interactions, the imphed rigidity of the... 

The LOCK AND KEY model for enzyme specificity uses complementarity between the enzyme active site (the lock) and the substrate (the key). Simply, the substrate must fit correctly into the active site—it must be the right size and shape, have charges in the correct place, have the right hydrogen-bond donors and acceptors, and have just the right hydrophobic patches. 

Two models currently exist to explain how an enzyme and its substrate interact. One model, called the lock and key model, suggests that an enzyme is like a lock, and its substrate is like a key. The shape of the active site on the enzyme exactly fits the shape of the substrate. A second model, called the induced fit model, suggests that the active site of an enzyme changes its shape to fit its substrate. Figure 6.21 shows both models. 

Diagram A shows the lock and key model of enzyme function. Diagram B shows the induced-fit model of enzyme function. 

One of the earliest, simplest, and best-known examples of this concept is the lock-and-key model of enzyme action hrst proposed by German chemist Emil Fischer (1852-1919) in 1894. While it has been significantly modihed since that time, the general mode of action suggested by Fischer is probably generally correct for most types of enzyme action. According to the lock-and-key model, illustrated below. 

The topologically defined region(s) on an enzyme responsible for the binding of substrate(s), coenzymes, metal ions, and protons that directly participate in the chemical transformation catalyzed by an enzyme, ribo-zyme, or catalytic antibody. Active sites need not be part of the same protein subunit, and covalently bound intermediates may interact with several regions on different subunits of a multisubunit enzyme complex. See Lambda (A) Isomers of Metal Ion-Nucleotide Complexes Lock and Key Model of Enzyme Action Low-Barrier Hydrogen Bonds Role in Catalysis Yaga-Ozav /a Plot Yonetani-Theorell Plot Induced-Fit Model Allosteric Interaction... 

LINKED FUNCTIONS LOCK-AND-KEY MODEL OF ENZYME ACTION... 

Liquid Crystals materials that have properties of both solids and liquids used extensively in digital displays Lithosphere outer surface of Earth including the crust and upper mantle Lock-and-Key Model model to explain how enzymes catalyze reactions with specific enzymes acting as locks that only certain substrates which act as keys can fit... 

According to the lock-and-key model, an enzyme is pictured as a large, irregularly shaped molecule with a cleft, or crevice, in its middle. Inside the crevice is an active site, a small region with the shape and chemical composition necessary to bind the substrate and catalyze the appropriate reaction. In other words, the active site acts like a lock into which only a specific key can fit (Figure 24.10). An enzyme s active site is lined by various acidic, basic, and neutral amino acid side chains, all properly positioned for maximum interaction with the substrate. 

FIGURE 24.10 According to the lock-and-key model, an enzyme is a large, three-dimensional molecule containing a crevice with an active site. Only a substrate whose shape and structure are complementary to those of the active site can fit into the enzyme. The active site of the enzyme hex-ose kinase is visible as the cleft on the left in this computergenerated structure, as is the fit of the substrate (yellow) in the active site. 

Early enzymatic theory emphasized the importance of high complementarity between an enzyme s active site and the substrate. A closer match was thought to be better. This idea was formally described in Fischer s lock and key model. The role of an enzyme (E), however, is not simply to bind the substrate (S) and form an enzyme-substrate complex (ES) but instead to catalyze the conversion of a substrate to a product (P) (Scheme 4.2). Haldane, and later Pauling, stated that an enzyme binds the transition state (TS ) of the reaction. Koshland expanded this theory in his induced fit hypothesis.5 Koshland focused on the conformational flexibility of enzymes. As the substrate interacts with the active site, the conformation of the enzyme changes (E — E ). In turn, the enzyme pushes the substrate toward its reactive transition state (E TS ). As the product forms, it quickly diffuses out of the active site, and the enzyme assumes its original conformation. 

Enzymes owe their superb adivity and seledivity to the spatial and chemical configuration of the adive site. The enzyme cavity fits around the substrate (or substrates), and the multipoint contad directs it precisely to the desired reaction center. The lock-and-key model, introduced in 1894 by the German chemist and 1902 Nobel laureate Emil Fischer [17], is an excellent analogy (Figure 5.3a). This model was... 

The active site is the region of the enzyme that binds the substrate, to form an enzyme-substrate complex, and transforms it into product. The active site is a three-dimensional entity, often a cleft or crevice on the surface of the protein, in which the substrate is bound by multiple weak interactions. Two models have been proposed to explain how an enzyme binds its substrate the lock-and-key model and the induced-fit model. 

Binding of a substrate to an enzyme, (a) Lock-and-key model (b) induced-fit model. 

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