Lyase, definition

K. Trummler, H. Wajant, Molecular cloning of acetone cyanohydrin lyase from flax Linum usitatissimum). Definition of a novel class of hydroxynitrile lyases./. Biol. Chem. 1997, 272, 4770-4774. 

Because of coupling (see Chapter 7) there are relationships between the thermodynamic properties of reactions in some of the EC classes. All oxidoreductase reactions can be considered to be coupled reactions because each one can be divided into two, or in a some cases, three half reactions that do not share atoms but are connected by formal electrons. Transferase reactions can each be considered to result from the coupling of two oxidoreductase reactions or two hydrolase reactions. Fifteen examples are discussed in reference (6). Each of the coupled reactions contributes its A, G ° and A, A h to the coupled reaction. Hydrolase reactions and isomerase reactions are never coupled reactions. Some lyase reactions are coupled. Ligase reactions are all coupled by definition because they join together two reactions with the hydrolysis of a pyrophosphate bond in ATP or a similar triphosphate. A spectacular example of coupling is provided by EC 6.3.5.4 because there are seven reactants. This never happens in chemistry. 

FIGURE 19.13 Transfer of the starting materials of lipid anabolism from the mitochondrion to the cytosol. (1 is ATP-citrate lyase other symbols are as in Figure 19.10.) It is not definitely established whether acetyl-CoA is transported from the mitochondrion to the cytosol. 

Other examples of M.c. are anthranilate synthase and tryptophan synthase, which are involved in microbial Aromatic biosynthesis (see), and citrate lyase. The definition of M. c. may be extended to include the membrane-bound respiratory chain, the contractile protein complexes of muscle, and the ribosome (which also contains RNA), etc. 

While the results of the growth reversal experiments in the "supercomplex systems" described above had stimulated investigations at the level of "defined systems" (i.e. the examination of certain metabolic reactions in cell-free systems), no definitive answers as to the biochemical mode of action of glyphosate had been obtained. Based on our experience with inhibitors of phenylalanine ammonia-lyase (see below), we included the "complex system" level (i.e. the examination of a metabolic pathway jsi vivo) in our strategy in order to define the limits more closely. Hypocotyls from etiolated buckwheat seedlings provided a system in which the rapid synthesis of phenylalanine-derived products, such as anthocyanin and other phenylpropanoid compounds, can be very simply induced by illumination. Anthocyanins, in particular, can be conveniently extracted and quantified and, at least in buckwheat, are not subject to measurable turnover within... 

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