Metabolism kinase

Minerals such as Mg " ", Mn +, K+, and Na+ are necessary. The first two are often used as key enzyme cofactors of the metabolism (kinases, malolactic enzyme). The following trace elements are involved in the nutrition of lactococci Cu +, Fe " ", Mo and Se. Yet the role of these metal... 

The biological transformations that involve ATP are both numerous and funda mental They include for example many phosphorylation reactions m which ATP trans fers one of its phosphate units to the —OH of another molecule These phosphoryla tions are catalyzed by enzymes called kinases An example is the first step m the metabolism of glucose... 

In humans, thiamine is both actively and passively absorbed to a limited level in the intestines, is transported as the free vitamin, is then taken up in actively metabolizing tissues, and is converted to the phosphate esters via ubiquitous thiamine kinases. During thiamine deficiency all tissues stores are readily mobilhed. Because depletion of thiamine levels in erythrocytes parallels that of other tissues, erythrocyte thiamine levels ate used to quantitate severity of the deficiency. As deficiency progresses, thiamine becomes indetectable in the urine, the primary excretory route for this vitamin and its metaboHtes. Six major metaboHtes, of more than 20 total, have been characterized from human urine, including thiamine fragments (7,8), and the corresponding carboxyHc acids (1,37,38). 

FIGURE 23.22 The metabolic effects of insulin. As described in Chapter 34, binding of insulin to membrane receptors stimulates the protein kinase activity of the receptor. Subsequent phosphorylation of target proteins modulates the effects indicated. 

Group II assays consist of those monitoring cellular second messengers. Thus, activation of receptors to cause Gs-protein activation of adenylate cyclase will lead to elevation of cytosolic or extracellularly secreted cyclic AMP. This second messenger phosphorylates numerous cyclic AMP-dependent protein kinases, which go on to phosphorylate metabolic enzymes and transport and regulatory proteins (see Chapter 2). Cyclic AMP can be detected either radiometrically or with fluorescent probe technology. 

Unlike classical neurotransmitters, adenosine does not have a rapid synaptic uptake system (as for the biogenic amines), and its chemical inactivation system is not as rapid as for the transmitter acetylcholine, for example. Adenosine may be metabolized extracellularly and inactivated with respect to the ARs in a more general fashion by the widespread enzymes adenosine kinase (AK, to produce AMP) and adenosine deaminase (AD, to produce inosine). Both AMP and inosine are only weakly active at ARs, depending on the subtype. 

AMP-activated Protein Kinase. Table 2 Metabolic effects of AMPK activation. In cases marked with an asterisk, there is evidence that AMPK mediates its effects by modulating the target named, although it is not yet clear whether the protein is directly phosphorylated by AMPK... 

Stimulation of the insulin receptor results in the activation of two major pathways [3] (i) the mitogen-activated protein (MAP) kinase cascade (discussed in chapter MAP kinase cascade) and (ii) the phospha-tidylinositol 3-kinase (PI 3-kinase) pathway which has been extensively studied in the context of the metabolic responses to insulin (summarized in Table 1 and Fig. 2). 

Phosphorylation is the reversible process of introducing a phosphate group onto a protein. Phosphorylation occurs on the hydroxyamino acids serine and threonine or on tyrosine residues targeted by Ser/Thr kinases and tyrosine kinases respectively. Dephosphorylation is catalyzed by phosphatases. Phosphorylation is a key mechanism for rapid posttranslational modulation of protein function. It is widely exploited in cellular processes to control various aspects of cell signaling, cell proliferation, cell differentiation, cell survival, cell metabolism, cell motility, and gene transcription. 

Protein kinase A (PKA) is a cyclic AMP-dependent protein kinase, a member of a family of protein kinases that are activated by binding of cAMP to their two regulatory subunits, which results in the release of two active catalytic subunits. Targets of PKA include L-type calcium channels (the relevant subunit and site of phosphorylation is still uncertain), phospholam-ban (the regulator of the sarcoplasmic calcium ATPase, SERCA) and key enzymes of glucose and lipid metabolism. 

Transduction mechanism Inhibition of adenylyl cyclase stimulation of tyrosine phosphatase activity stimulation of MAP kinase activity activation of ERK inhibition of Ca2+ channel activation stimulation of Na+/H+ exchanger stimulation of AM PA/kainate glutamate channels Inhibition of forskol in-stimulated adenylyl cyclase activation of phos-phoinositide metabolism stimulation of tyrosine phosphatase activity inhibition of Ca2+ channel activation activation of K+ channel inhibition of AM PA/ kainate glutamate channels inhibition of MAP kinase activity inhibition of ERK stimulation of SHP-1 and SHP-2 Inhibition of adenylyl cyclase stimulation of phosphoinositide metabolism stimulation of tyrosine phosphatase activation of K+ channel inhibi-tion/stimulation of MAP kinase activity induction of p53 and Bax Inhibition of adenylyl cyclase stimulation of MAP kinase stimulation of p38 activation of tyrosine phosphatase stimulation of K+ channels and phospholipase A2 Inhibition of adenylyl cyclase activation/ inhibition of phosphoinositide metabolism inhibition of Ca2+ influx activation of K+ channels inhibition of MAP kinase stimulation of tyrosine phosphatase... 

It is generally accepted that the two main fiber types have different metabolic profiles with higher activities of Ca activated myosin ATPase, creatine kinase. 

All NRTIs, as exemplified for AZT (Fig. 7), act in a similar fashion following their uptake by the cells, they are phosphorylated successively to their 5 -monophosphate, 5 -diphosphate, and 5 -triphosphate form (De Clercq 2002). Unlike the first phosphorylation step in the metabolic pathway of the acyclic guanosine analogues (see above), which is carried out by a virus-encoded enzyme (thymidine kinase), the first as well as the subsequent phosphorylations of the 2, 3 -dideoxynucleosides are carried out by cellular enzymes, that is, a 2 -deoxynucleoside (e.g., dThd) kinase, a 2 -deoxynucleotide (e.g., dTMP) kinase, and a (2 -deoxy)nucleoside 5 -diphosphate (NDP) kinase. 

Entecavir, telbivudine, clevudine, and the other nucleoside analogues (Fig. 4aa) need to be phosphorylated to their 5 -triphosphate form to be antivirally active (Fig. 8). This again implies three phosphorylation steps based successively on a nucleoside kinase, nucleoside 5 -monophosphate kinase, and nucleoside 5 -diphosphate kinase. These reactions have been characterized only in a few cases, that is, thymidylate kinase in the metabolism of clevudine (Hu et al. 2005). 

---