Inactive Form of an Enzyme

Similar examples of immunochemical isolation of inactive proteins may be found in the isolation of a precursor protein. Such situations are well documented for the zymogen systems such as trypsinogen-trypsin, chy-motrypsinogen-chymotrypsin, and proinsulin-insulin. 

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There are two general types of covalent modification of enzymes that regulate their activity. These are the irreversible activation of inactive enzyme precursors, the zymogens, and the reversible interconversion of active and inactive forms of an enzyme. 

A zymogen, or proenzyme, is the inactive form of an enzyme that is converted to the active form of the enzyme at the site of its activity. 

The matrix metalloprotease (MMP) family of zinc hydrolases are thought to play important roles in extracellular tissue remodeling in angiogenesis and other normal physiological processes, in some inflammatory processes and in metastatic processes in cancer. Like the zinc carboxypeptidases, the MMPs also utilize a zinc-coordinated water molecule to initiate attack on the scissile amide bond of protein substrates. These enzymes are synthesized by the ribosome in a latent form composed of a catalytic domain and an N-terminal extension, referred to as the prodomain the latent, or inactive form of the enzyme is referred to as a zymogen or... 

The location of NADPH in the lateral channel of small-subunit enzymes blocks the channel such that it cannot realistically be considered to have either an inlet or exhaust function in the presence of the nucleotide. For the channel to have a role in these enzymes, it is necessary to predict that NADPH binds preferentially to the enzyme in its resting state or to inactive forms of the enzyme, such that NADPH is not bound when the enzyme is actively degrading H2O2. Thus, the variation in NADPH occupancy among enzymes might reflect the reactive history of the enzyme immediately prior to isolation, as well as the Kn for NADPH binding and the NADPH concentration. For example, enzyme isolated from a culture with high peroxide levels may have a lower amounts of bound NADPH because the enzyme is more active. 

Aconitase contains iron in the form of an Fe4S4 iron-sulfur cluster (Fig. 13-4).81-83 However, the enzyme is usually isolated in a form that does not show its maximum activity until it has been incubated with ferrous iron (Fe2+). The inactive form of the enzyme is thought to contain an Fe3S4 cluster (Chapter 16) which is converted back to the Fe4S4 cluster by the incubation... 

An alternative way of viewing the induced-fit process is to divide it into hypothetical steps. We can suppose that there is an equilibrium between the inactive form of the enzyme, Ein, which is the major species ([Ein] [E]0), and a small fraction of active enzyme, Eact, in which the catalytic groups are correctly aligned ... 

It is possible to devise kinetic curiosities that give Michaelis-Menten kinetics without the enzyme being saturated with the substrate. For example, in the following scheme—where the active form of the enzyme reacts with the substrate in a second-order reaction to give the products and an inactive form of the enzyme, E, which slowly reverts to the active form—apparent saturation kinetics are followed with cat = k2 and Ku = k2/k1. Equation 3.24 applies to this example if E is treated as a bound form of the enzyme ... 

Zymogen. An inactive precursor of an enzyme. For example, trypsin exists in the inactive form trypsinogen before it is converted to its active form, trypsin. 

The catalytic dismutation of superoxide is actually more complicated in E. coli [42] and B. thermophilus [43] Mn-SODs than that of either Cu or Fe proteins since it may involve an inactive form of the enzyme. The inactive form is believed [44] to contain a Mnm-side-on peroxo unit (of the type shown in Figure 29) formed within the hydrophobic environment of MnSOD, in the absence of H+, by the oxidative addition of the superoxide ion to the Mn11 center. When H+ ions are present, an active, end-on peroxo complex forms, yielding successively a bound hydroperoxide ion and free dihydrogen peroxide (cf. Figure 3). Thus, the key parameter that turns the reaction off or on may be the absence or presence of a H+ ion [44],... 

The lipoxygenase family of enzymes catalyse stereospecific oxygenation reactions of fatty-acid substrates. The active site incorporates a non-haem iron centre in as yet an unidentified environment. The active form of the enzyme is the iron(III) state but is converted to iron(II) as the fatty acid is oxidised [91]. Catechol-containing compounds inhibit these enzymes by forming a ternary complex with the iron centre and depending on the substituents on the catechol moiety may even facilitate an internal redox reaction leading to the formation of iron(II) and an inactive form of the enzyme [92,93]. 

Enzyme Cofactors- In many enzymatic reactions, and in particular biological reactions, a second substrate (i.e., species) must be introduced to activate the enzyme. This substrate, which is referred to as a cofactor or coenzyme even though it is not an enzyme as such, attaches to the enzyme and is most often either reduced or oxidized during the course of die reaction. The enzyme-cofactor complex is referred to as a holoenzyme. The inactive form of the enzyme-cofactor complex for a specific reaction and reaction direction is called an apoenzyme. An example of the type of system in which a cofactor is used is the formation of ethanol from acetaldehyde in the presence of the enzyme alcohol dehydrogenase (ADH) and the cofactor nicotinamide adenine dinuoleotide (NAD) ... 

Adrenaline exerts its effect by binding to a receptor site on the cell surfaces of liver and muscle cells, where it initiates a series of signals that ultimately causes an inactive form of the enzyme glycogen phosphorylase to become active. This enzyme is the first in a sequence that leads to the breakdown of glycogen to glucose and other products. 

Human pancreatic lipase is a protein consisting of 445 amino acids. It folds in two domains with the larger N-terminal containing the catalytic site. The overall folding is represented in a ribbon-diagram in Fig. 17. This crystallographic structure of the human pancreatic lipase represents an inactive form of the enzyme [17]. The active site is totally buried by a loop and... 

Since the structure observed in the crystals represents an inactive form of the enzyme, several structural modifications were required to accommodate a substrate molecule in the active site. These modifications were performed such that the resulting model remained as similar as possible to the experimental structure. First, the loop covering the active site which is anchored at a disulfide bridge was shifted to an alternative strainless conformation opening up the active site. Second, the side-chain orientations of three aromatic residues near the active site had to be changed (Fig. 19). Third, the segment to the right of the active site had to be... 

In summary, the active site structure of the oxidized diiron phosphatases remains a puzzle because of the seemingly conflicting experimental observations. In of itself, the diferric phosphatases represent an inactive form of these enzymes and are thus of less biochemical interest than the active Fe(III)Fe(II) and Fe(III)Zn(II) forms. The details of the diferric active site are intriguing from a bioinorganic perspective, however, mainly in terms of its relationship to the active sites of Hr, MMO, and RRB2. 

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