Amino acid side chains in or near the binding site can then act as acid or base catalysts, provide binding sites for the transfer of functional groups from one substrate to another or aid in the rearrangement of a substrate. The structural changes that occur when an enzyme and a substrate join together bring specific parts of a substrate into alignment with specific parts of the enzyme’s active site. (b) The enzyme conformation changes dramatically when the substrate binds to it, resulting in additional interactions between hexokinase and glucose. (a) The enzyme hexokinase without its substrate (glucose, shown in red) is bound to the active site. After catalysis, the enzyme resumes its original structure.įigure 18.12 The Induced-Fit Model of Enzyme Action The current theory, known as the induced-fit model A model that says an enzyme can undergo a conformational change when it binds substrate molecules., says that enzymes can undergo a change in conformation when they bind substrate molecules, and the active site has a shape complementary to that of the substrate only after the substrate is bound, as shown for hexokinase in Figure 18.12 "The Induced-Fit Model of Enzyme Action". They discovered that the binding of a substrate often leads to a large conformational change in the enzyme, as well as to changes in the structure of the substrate or substrates. Working out the precise three-dimensional structures of numerous enzymes has enabled chemists to refine the original lock-and-key model of enzyme actions. (b) The catalytic reaction occurs while the two are bonded together in the enzyme-substrate complex.
(a) Because the substrate and the active site of the enzyme have complementary structures and bonding groups, they fit together as a key fits a lock.
This model portrayed the enzyme as conformationally rigid and able to bond only to substrates that exactly fit the active site.įigure 18.11 The Lock-and-Key Model of Enzyme Action ( Figure 18.11 "The Lock-and-Key Model of Enzyme Action"). In fact, an early model describing the formation of the enzyme-substrate complex was called the lock-and-key model A model that portrays an enzyme as conformationally rigid and able to bond only to a substrate or substrates that exactly fit the active site. It possesses a unique conformation (including correctly positioned bonding groups) that is complementary to the structure of the substrate, so that the enzyme and substrate molecules fit together in much the same manner as a key fits into a tumbler lock. of the enzyme ( Figure 18.10 "Substrate Binding to the Active Site of an Enzyme").
This pocket, where the enzyme combines with the substrate and transforms the substrate to product is called the active site The location on an enzyme where a substrate binds and is transformed to product. The structural features or functional groups on the enzyme that participate in these interactions are located in a cleft or pocket on the enzyme surface. Hydrogen bonding and other electrostatic interactions hold the enzyme and substrate together in the complex.
LOCK AND KEY MODEL OF ENZYME ACTION FREE
(This step is reversible because the complex can break apart into the original substrate or substrates and the free enzyme.) Once the E–S complex forms, the enzyme is able to catalyze the formation of product (P), which is then released from the enzyme surface: S + E → E–S E–S → P + E In the first step, an enzyme molecule (E) and the substrate molecule or molecules (S) collide and react to form an intermediate compound called the enzyme-substrate (E–S) complex.
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