Enzymatic reactions cascade

An enzymatic reaction cascade can be defined as sequential activation of a series of enzymes triggered, for example, by ligand binding to a receptor and resulting... 

Clotting cascade A series of enzymatic reactions by clotting factors leading to the formation of a blood clot. The clotting cascade is initiated by several thrombogenic substances. Each reaction in the cascade is triggered by the preceding one, and the effect is amplified by positive feedback loops. 

Fig. 2. An adrenaline molecule (1) binds to its binding site on the extracellular site of an adrenaline receptor (2). Thereby, the exchange of GDP by GTP in the Ga subunit of a hetero-trimeric G protein (3) is induced, followed by the dissociation of the Ga and Gpr subunits. G now binds and stimulates its effector adenylate cyclase (4), which produces cyclic AMP (5) from ATP (6). This second messenger starts a cascade of enzymatic reactions, which alter the behavior of the cell via several phosphorylation steps... 

One of the established methods for finding hits is high-throughput screening (HTS). This is a powerful method enabling several thousands of compounds per day to be tested. However, there are drawbacks. In many cases the assay for the detection of a hit comprises coupled enzymatic reactions, and a test compound may falsely lead to positive results because of its interference with components of the enzyme cascade. Problems may arise further from the chemical substances that make up the compound depository which are tested in HTS. Especially for companies with historically grown collections of compounds, the quality of the chemicals is an uncontrolled parameter in the sense that it is not always known whether the compounds are still unmodified they may have decomposed, precipitated or formed aggregates (polymers). 

Normally, thrombin is present in the blood as an inactive proenzyme (see p. 270). Prothrombin is activated in two different ways, both of which represent cascades of enzymatic reactions in which inactive proenzymes (zymogens, symbol circle) are proteolytically converted into active proteinases (symbol sector of a circle). The proteinases activate the next proenzyme in turn, and so on. Several steps in the cascade require additional protein factors (factors 111, Va and Villa) as well as anionic phospholipids (PL see below) and Ca "" ions. Both pathways are activated by injuries to the vessel wall. 

Endotoxicity results from the interaction of a bacterial cell envelope component (e.g., LPS or PG with a cell surface receptor constituting part of the nonspecific immune system, (i.e., a toll-like receptor on white blood cells). This results in the production of cytokines [e.g., interleukin 1 (IL-1) or tumor necrosis factor (TNF)] as part of an intracellular enzyme cascade which can cause severe tissue injury. Bioassays or immunoassays can be used to detect such reactions respectively. As noted above the most widely used bioassay is the LAL assay. A lysate of amoebo-cytes of the horseshoe crab (Limulus) contains an enzymatic clotting cascade which is activated by extremely low levels of LPS (nanogram levels or lower). There are variants of this assay that can detect PG, but they are not as widely used. As noted above, other bioassays employ cultured cell lines that respond to LPS or PG, respectively. Unfortunately bioassays are highly amenable to false positives (from the presence of cross-reactive substances) or false negatives from inhibition (by contaminants present in the sample) [10]. A detailed discussion of these assays is beyond the scope of this chapter and has been reviewed elsewhere [1]. 

The second stage of this cascade takes place at the cytoplasmic surface of the bacterial cell membrane. In the first reaction, MraY catalyzes a pyrophosphate exchange reaction wherein the UDP-MurNAc-pentapeptide precursor is coupled to a membrane-anchored C55 lipid carrier with ejection of UMP to provide undecaprenylpyrophosphoryl-MurNAc-pentapeptide 4, also know as lipid I. In the second reaction, the MurG enzyme catalyzes the transfer of GlcNAc from a UDP-GlcNAc precursor to the C(4)-hydroxyl group of the lipid-linked MurNAc-pentapeptide. The product of this enzymatic reaction, lipid II5, is the... 

In several animal studies severe adverse effects of hypothermia were clotting abnormalities and coagulopathy (22,42). In baboons, systemic hypothermia led to increased bleeding times (43). In men the enzymatic reactions of the coagulation cascade were shown to be strongly inhibited by hypothermia (44,45). However, severe clotting abnormalities have... 

The nitrilase mediated DKR route to enantiomerically pure 2-hydroxycarboxylic acids is restricted to the (R)-enantiomers because, to our knowledge, no (S)-selec-tive nitrilases for cyanohydrin substrates are commonly available [11]. We reasoned that a fully enzymatic route to the (S)-acids should be possible by combining an (S)-selective oxynitrilase (hydroxynitrile lyase, EC 4.1.2.10, (S)-hydroxynitrile lyase) and a non-selective nitrilase in a bienzymatic cascade (see Figure 16.3). Besides being more environmentally acceptable than chemical hydrolysis, the mild reaction conditions of the combined enzymatic reaction would be compatible with a wide range of hydrolysable groups. 

Biocatalytic synthetic reactions also include carbon dioxide fixation with the production of methanol in artificial multi-enzyme systems [188]. Formate dehydrogenase (FDH, EC 1.2.1.2) can catalyze the reduction of carbon dioxide to formate, and methanol dehydrogenase (MDH, EC 1.1.99.8) can catalyze the reduction of formate to methanol. Both of these enzymes require NAD+-NADE1 cofactor, and in the presence of the reduced dimethyl viologen mediator (MV+), they can drive a sequence of enzymatic reactions. The cascade of biocatalytic reactions results in the reduction of CO2 to formate catalyzed by FDEI followed by the reduction of formate to methanol catalyzed by MDH. A more complex system composed of immobilized cells of Parococcus denitrificans has been demonstrated for the reduction of nitrate and nitrite [189]. 

This cooperation not only takes place between the sinusoidal cells, but also with the hepatocytes .) endothelial and Kupffer cells complement each other through various mechanisms of endocytosis and different pathways of enzymatic clearance (2.) as a double barrier, they protect the liver cells both from toxic and undesired substances, even to the extent of self-sacrifice (i.) they intervene in the metabolism of liver cells with self-produced substances 4.) they send signal substances like cytokines (e.g. interferons) and eicosanoids (e.g. leukotrienes) to the liver cells for independent control of biochemical and biomolecular reaction cascades. 

Fig. 23.7. Dynamics of an enzymatic reaction in lipid nanotube networks with variable topology numeric calculations (bottom)/fluorescence intensity of the reaction product (top) vs. time for three differently chosen network geometries, (a) Reference experiment a static four-vesicle network. The product concentration displays a cascade-like behavior in time and space, (b) Linear-to-circular topology change in the four-vesicle network (c) A model study of the effect of product inhibition as the linear four-vesicle network (top panel) undergoes the same change in structure (bottom panel) as the network in the reference experiment ([28], reprinted with permission)...

Mechanisms. cAMP plays a central role as a second messenger in the regulation of cell metabolism (14). Binding of cAMP with the regulatory subunit of a protein kinase initiates a cascade of enzymatic reactions that ultimately lead to the breakdown of glycogen and release of glucose to the blood stream. The intracellular concentration of cAMP represents a balance between the action of adenylate cyclase (which produces cAMP... 

Restoration of blood flow following cardiac arrest can lead to several chemical cascades and destructive enzymatic reactions that can result in cerebral injury. These reactions include free-radical production, excitatory amino acid release, and calcium shifts, leading to mitochondrial damage and apoptosis (programmed cell death). Hypothermia can protect from cerebral injury by suppressing these chemical reactions, thereby reducing the production of free radicals. Various animal models have demonstrated improved functional recovery and reduced cerebral deflcits with the induction of mild therapeutic hypothermia. Recently, there have been two clinical trials in humans evaluating this technique. ... 

The coagulation cascade is a stepwise series of enzymatic reactions that results in the formation of a fibrin mesh. " Clotting factors... 

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