Enzymes pathway regulation

Regulation of the overall flux through a pathway is important to ensure an appropriate supply, when required, of the products of that pathway. Regulation is achieved by control of one or more key reactions in the pathway, catalyzed by regulatory enzymes. The physicochemical factors that control the rate of an... 

Fig. 5. The metabolic pathway for glycolysis and gluconeogenesis. (A) An illustration of overall reactions among key compounds. (B) The enzymes responsible for the reactions are denoted in rectangles with the EC numbers inside. The two shaded enzymes are key enzymes that regulate the overall direction of glycolysis or gluconeogenesis.

To provide a mechanism for the feedback inhibition of these enzymes, the allosteric model was put forward in 1963. It was proposed that the enzyme that regulates the flux through a pathway has two distinct binding sites, the active site and a separate site to which the regulator binds. This was termed the allosteric site. The word allosteric means different shape , which in the context of this mechanism means a different shape from the substrate. The theory further proposed that when the regnlator binds to the allosteric site, it canses a conformational change in... 

The next quantitative problem is to understand the basic mechanism of interaction between the regulator (in this case x) and its binding to the target enzyme (i.e. the enzyme that regulates the flux through the pathway). 

Metabolite flow along a metabolic pathway is mainly determined by the activities of the enzymes involved (see p. 88). To regulate the pathway, it is suf cient to change the activity of the enzyme that catalyzes the slowest step in the reaction chain. Most metabolic pathways have key enzymes of this type on which the regulatory mechanisms operate. The activity of key enzymes is regulated at three independent levels ... 

Phospholipase C, which occurs in different subtypes in the cell, is a key enzyme of phosphatide inositol metabohsm (for cleavage specificity, see Fig. 5.24). Two central signaling pathways regulate phosphohpase C activity of the cell in a positive way (Fig. 6.4). Phospholipases of type CP (PL-CP) are activated by G-proteins and are thus linked into signal pathways starting from G-protein-coupled receptors. Phosphohpases of type Y (PL-Cy), in contrast, are activated by transmembrane receptors with intrinsic or associated tyrosine kinase activity (see Chapter 8, Chapter 10). The nature of the extracellular stimuli activated by the two major reaction pathways is very diverse (see Fig 6.4), which is why the phosphohpase C activity of the cell is subject to multiple regulation. 

Allosteric enzymes are regulated by molecules called effectors (also modifiers) that bind noncovalently at a site other than the active site. These enzymes are composed of multiple subunits, and the regula tory site that binds the effector may be located on a subunit that is not itself catalytic. The presence of an allosteric effector can alter the affinity of the enzyme for its substrate, or modify the maximal cat alytic activity of the enzyme, or both. Effectors that inhibit enzyme activity are termed negative effectors, whereas those that increase enzyme activity are called positive effectors. Allosteric enzymes usually contain multiple subunits, and frequently catalyze the commit ted step early in a pathway. 

Inhibition of the initial step of a biosynthetic pathway by an end product of the pathway is a recurrent theme in metabolic regulation. In addition, many key enzymes are regulated by ATP, adenosine diphosphate (ADP), AMP, or inorganic phosphate ion (Pi). The concentrations of these materials provide a cell with an index of whether energy is abundant or in short supply. Because ATP, ADP, AMP, or P often are chemically unrelated to the substrate of the enzyme that must be regulated, they usually bind to an allosteric site rather than to the active site. 

Allosteric effectors are inhibitors or activators that bind to enzymes at sites distinct from the active sites. Allosteric regulation allows cells to adjust enzyme activities rapidly and reversibly in response to changes in the concentrations of substances that are structurally unrelated to the substrates or products. The initial steps in a biosynthetic pathway commonly are inhibited by the end products of the pathway, and numerous enzymes are regulated by ATP, ADP, or AMP. 

DMP accounts for all important aspects of endogenous biochemistry/biology that influence substrate selection and metabolism. For example, although the lipid 2-oleoyl glycerol is an excellent FAAH substrate in vitro, other enzymes and pathways regulate this lipid in vivo and, as a result, the loss of FAAH activity is inconsequential to the in vivo levels of this metabolite [5]. 

All the major biosynthetic pathways use acetyl-CoA as the basic building block, and in each pathway the rate limiting enzyme is regulated by phosphorylation with the phosphorylated enzyme being active. In the biosynthesis of cholesterol, the rate limiting step is catalyzed by hydroxymethylglutaryl-CoA (HMG-CoA) reductase. Initially, three molecules of acetyl-CoA are condensed to produce /5-HMG-CoA. HMG-CoA reductase then uses two NADPH molecules to reduce HMG-CoA to mevalonate-CoA. The remaining steps in cholesterol biosynthesis are numerous and well-documented. 

Understand enzyme regulation via cellular enzyme levels, compartmentation, metabolic pathway regulation, and covalent modifications. 

There are two major ways of control. One mechanism involves reversible covalent modifications, such as phosphorylation dephosphorylation, the other requires conformational transitions by binding an allosteric ligand or regulator protein. It follows an example of regulation of an enzyme, of which the activity is subject to control by both mechanisms, then we compare the regulation of an enzyme with regulation of components of cellular signalling pathways, of which many have no enzymic activity. 

A variety of phytoplankton are known to possess cell surface oxidases (Palenik and Morel, 1990a,b) and extracellular oxidation of amino acids has been shown to occur in nature (Pantoja and Lee, 1994 Mulholland et al., 1998, 2002a). Direct uptake of amino acid-derived N from this process represented up to 4% of the observed NFI4+ uptake in a mid-Adantic estuary (Mulholland et al., 2003). Recently, a cell surface protein expressed under N-limitation was identified as a deaminase suggesting that these enzymes are regulated by cellular N status as for other pathways of N uptake and metabolism (Palenik and Koke, 1995). Failure to account for alternative pathways for mobilization of DON might result in underestimates of its utilization in nature. 

---