Mechanisms of regulation

Enzymatic activities. The hydrolysis of ATP by actin-activated myosin is the characteristic enzymatic activity of muscle, smooth muscle included. All forms of smooth muscle myosin are slower than those of other muscles. The binding site for ATP and a reduced enzymatic activity are still present in monomeric myosin. The enzymatic activity of monomeric myosin is altered by a conformational change, (the 10S-6S transition) and the species of cations present in the reaction mixture. These differences relate to the possible mechanisms of regulation. 

The contraction of muscles from all sources occurs by the general mechanism described above. Muscles from different organisms and from different cells and tissues within the same organism may have different molecular mechanisms responsible for the regulation of their contraction and relaxation. In all systems, plays a key regulatory role. There are two general mechanisms of regulation of muscle contraction actin-based and myosin-based. The former operates in skeletal and cardiac muscle, the latter in smooth muscle. 

Ca2+ can enter cells via voltage- or ligand-dependent channels and by capacitative entry. These three fundamental mechanisms of regulated calcium ion entry across the plasma membrane involve, respectively, voltage-dependent Ca2+ channels, ligand-gated Ca2+ channels and capacitative Ca2+ entry associated with phospholipase C-coupled receptors. 

Viarengo, A. (1989). Heavy metals in marine invertebrates mechanisms of regulation and toxicity at the cellular level, Rev. Aquat. Sci., 1, 295-317. 

Rossmann, R., Sawers, G. and Bock, A. (1991) Mechanism of regulation of the formate-hydro-genlyase pathway by oxygen, nitrate, and pH Definition of the formate regulon. Mol. Microbiol., 5, 2807-14. 

Matkovits T, Christakos S. 1995. Variable in vivo regulation of rat vitamin D-dependent genes (osteopontin, Ca, Mg-adenosine triphosphatase, and 25-hydroxyvitamin D3 24-hydroxylase) implications for differing mechanisms of regulation and involvement of multiple factors. Endocrinology 136 3971-3982. 

In any cell that depends on aerobic metabolism, if the rate of ATP utilisation increases, the rate of the Krebs cycle, electron transfer and oxidative phosphorylation must also increase. The mechanism of regulation discussed here is for mammalian skeletal muscle since, to provide sufficient ATP to maintain the maximal power output, at least a 50-fold increase in flux through the cycle is required so that the mechanism is easier to study (Figure 9.22). 

Figure 11.30 Mechanisms of regulation of phospholipase A2. In all these processes described above, it is phospholipase A that carries out the hydrolysis of membrane phospholipid. Cytokines are local hormones produced by immune cells, T-lymphocytes and macrophages (Chapter 17). Other factors relate to shear stress in endothelial cells and those that stimulate release of granules from mast cells. Eicosanoids are present in the granules and they must be re-synthesised after degranulation in the mast cells. Here the enzymes described above must be present in mast cells.

The procedures for studying mechanisms of regulation of the rates of biochemical processes are described in Chapter 3. These procednres are applied to peptide synthesis in this discussion. There are three qnestions that must be answered prior to discussion of mechanisms, (i) Which reactions are non-eqnilibrium (ii) Which is the flnx-gen-erating reaction and how is it regulated (iii) What reactions other than the flnx-generating steps are regulated by external factors, i.e. external to the intermediates in peptide synthesis ... 

The initiation factor proteins regulate the formation of the complex between a tRNA-amino acid, the 40S ribosome and mRNA. There are at least ten initiation factors but the roles of all these factors are not known. One key factor is usually abbreviated to eIF2 but is abbreviated to IF in the present discussion. The mechanism of regulation of the... 

Figure 20.27 A simplified description of the mechanism of regulation of the activity of the initiation factor for initiation of peptide synthesis.

As might be expected from other mechanisms of regulation described in this text, phosphorylation and dephosphorylation of key proteins is the main mechanism for regulating the cycle, i.e. reversible phosphorylation, also known as interconversion cycles (discussed in Chapter 3). In the cell cycle, several of these interconversion cycles play a role in control at the checkpoints. Two important terms must be appreciated to help understand the mechanism of regulation of the cycle the phosphorylation of proteins is catalysed by specific protein kinases, known as cell-division kinases (cdck) or cell cycle kinases (cck) and these enzymes are activated by specific proteins, known as cyclins. 

Mechanism of regulation of cell cycle kinases and activation of checkpoints... 

Figure 80. Mechanism of regulation of alkaloid content in plants. Abbreviations RS - regulation system OH - overhigh level OL - overlow level LGF - life growing factors. Observe that this regulation system is coded in genes. Life growing factors (light, water, COj, nutrients including nitrogen, temperature, etc.) influence on stress, which is also dependent on this system.

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