Proposed Regulatory Mechanisms

TABLE II General Mechanisms for Regulating Enzyme Systems 

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The RyR channels seem to be regulated by luminal Ca2+. Luminal Ca2+ may activate theRyR2 channels in the heart. The association of calsequestrin with RyR2 via triadin or junctin is proposed as a possible regulatory mechanism. Such activation by luminal Ca2+ remains controversial in the skeletal muscle. 

U.S. EPA proposed to significantly impact the RCRA hazardous waste identification process through a rulemaking effort called the Hazardous Waste Identification Rules (H WIR). The first rule, HWIR-media, was finalized on November 30,1998, and addressed contaminated media.16 The second rule, HWIR-waste, was finalized on May 16, 2001, and modified the mixture and derived-from rules, as well as the contained-in policy for listed wastes.5 Both the HWIR-media rule and the HWIR-waste rule attempt to increase flexibility in the hazardous waste identification system by providing a regulatory mechanism for certain hazardous wastes with low concentrations of hazardous constituents to exit the RCRA Subtitle C universe. 

Apart from the phosphorylation theory, other regulatory mechanisms have also been suggested for smooth muscle contraction. A thin-filament protein that has been proposed as a regulatory component is caldesmon [102], Purified caldesmon is a potent inhibitor of actin-tropomyosin interaction with myosin. The mechanisms by which calcium removes this inhibition are controversial. Furthermore, phosphorylation of caldesmon by a caldesmon kinase in vitro has also been implicated in this... 

It has been proposed that LHC phosphorylation-dephosphorylation is a regulatory mechanism to adjust any imbalance between PS 11 and PS I photochemical activities. When PS II prevails, the PQ pool becomes over-reduced, the kinase is activated, then LHC is phosphorylated and more excitation energy flows to PS I. Oxidation of PQH2 follows, then inactivation of the kinase the phosphatase (for this enzyme no regulation has been reported) will then dephosphorylate LHC [127-129],... 

Fig. 8.22 Regulation of protein tyrosine phosphatases (PTPs). The specific activity of PTPs can be either increased or decreased by phosphorylation of serine, threonine or tyrosine residues. Reversible or irreversible oxidation of the active-site cysteine residue (denoted C) also inactivates PTPs. For receptor-like PTPs (RPTPs), dimerization of the catalytic domains has also been proposed as an inhibitory regulatory mechanism. Furthermore, ligand binding to the extracellular domains has been proposed as an inhibitory mechanism. Modulation of specific activity following ligand binding could involve changes in the phosphorylation, oxidation state of the active-site cysteine or dimerization.

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