Phosphorylation initiated transition

Insulin binding to the extracellular side of cell membranes initiates the insulin cascade , a series of phosphorylation/dephosphorylation steps. A postulated mechanism for vanadium is substitution of vanadate for phosphate in the transition state structure of protein tyrosine phosphatases (PTP).267,268 In normal physiological conditions, the attainable oxidation states of vanadium are V111, Viv and Vv. Relevant species in solution are vanadate, (a mixture of HV042-/ H2VOO and vanadyl V02+. Vanadyl is not a strong inhibitor of PTPs, suggesting other potential mechanisms for insulin mimesis for this cation. 

The Gi-S transition (not shown) is particularly important for initiating the cell cycle, it is triggered by the CDK4-cyclin D complex, which by phosphorylating the protein pRb releases the transcription factor E2F previously bound to pRb. This activates the transcription of genes needed for DNA replication. 

Fig. 1.32. Phosphorylation of the C-terminal domain of RNA polymerase II and the beginning of transcription. The transition from the initiation complex to actual begin of transcription is regulated via phosphorylation of the C-terminal domain (CTD) of RNA polymerase II. In the above model it is assumed that initially a complex is formed between TFIID and a holoenzyme of RNA polymerase consisting of RNA polymerase II and associated factors (mediators, SRB proteins) and the basal transcription factors. Phosphorylation of the C-terminal domain effects the dissociation of the RNA polymerase from the initation complex and the transition to the elongation phase. A protein kinase, which is part of TFIIH, is responsible for the phosphorylation. The nature of the signal that induces phosphorylation of RNA polymerase II remains unknown. SRB suppressor of RNA polymerase B. After Koleske and Young (1995).

An example for how protein phosphorylation can influence the transcription process is the transition from the initiation to the elongation process for RNA polymerase II (see 1.4.2.4). 

DNA polymerases catalyze DNA synthesis in a template-directed manner (Box 16). For most known DNA polymerases a short DNA strand hybridized to the template strand is required to serve as a primer for initiation of DNA synthesis. Nascent DNA synthesis is promoted by DNA polymerases by catalysis of nucleophilic attack of the 3 -hydroxyl group of the 3 -terminal nucleotide of the primer strand on the a-phosphate of an incoming nucleoside triphosphate (dNTP), leading to substitution of pyrophosphate. This phosphoryl transfer step is promoted by two magnesium ions that stabilize a pentacoordinated transition state by complex-ation of the phosphate groups and essential carboxylate moieties in the active site (Figure 4.1.1) [2],... 

Catalysis can be broken down into a number of areas, depending on the substrate and the catalytic reaction. One of the prime areas of the initial effort in catalysis has been small molecule activation, such as oxygen with a number of transition metal ion macrocycles and carbon dioxide, the latter particularly with cobalt(I) and nickel(I) macrocycles. Once the polyanunonium macrocycles were found to be able to recognize substrates other than metal ions, other catalysis applications evolved. For example, phosphoryl transfer catalysis with simple polyanunonium macrocycles has become quite accessible. ... 

Figure 29.34 The CTD Coupling transcription to pre-mRNA processing. The transcription factor TFIIH phosphorylates the car boxy I-terminal domain (CTD) of RNA polymerase II, signaling the transition from transcription initiation to elongation. The phosphorylated CfD binds factors required for pre-mRNA capping, splicing, and polyadenyiation. These proteins are brought in close proximity lo their sites of action on the nascent pre-mRNA as it is transcribed during elongation.

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