Cellular energy status

Clearly, the activity of phosphofructokinase depends both on ATP and AMP levels and is a function of the cellular energy status. Phosphofructokinase activity is increased when the energy status falls and is decreased when the energy status is high. The rate of glycolysis activity thus decreases when ATP is plentiful and increases when more ATP is needed. 

At least two enzymes compete for acetyl-CoA - the citrate synthase and 3-ke-tothiolase. The affinities of these enzymes differ for acetyl-CoA (Table l),and at low concentrations of it the citrate synthase reaction tends to dominate, provided that the concentration of 2/H/ is not inhibiting. The fine regulation of the citrate synthases of various poly(3HB) accumulating bacteria has been studied [ 14, 47, 48]. They appear to be controlled by cellular energy status indicators (ATP, NADH, NADPH) and/or intermediates of the TCA cycle. The 3-ketothio-lase has also been investigated [10-14,49, 50]. This enzyme is, above all, inhibited by CoASH [10,14,49]. This important feature will be further considered below. 

Hardie, D. G. 2003. Minireview the AMP-activated protein kinase cascade the key sensor of cellular energy status. Endocrinology 144 5179-5183. 

Both the overall rate of protein synthesis and the translation of certain specific mRNAs are controlled by agents such as hormones, growth factors, and other extracellular stimuli. As precursors for protein assembly, amino acids also regulate the translational machinery. Because protein synthesis consumes a high proportion of cellular metabolic energy, the energy status of the cell also modulates translation factors. 

The nature and concentration of carbon and nitrogen substrates in the culture medium affect both the energy status and metabolism of the cells. Medium generally contains both a carbohydrate (normally glucose) and a range of amino acids to satisfy the cellular growth requirements (Butler, 2004). Lipids are also important, not only as an energy source, but also as precursors for the synthesis of the cellular membrane components. 

ATP is the cellular energy currency (Chapter 14). The use of this compound as a phosphoryl group donor links the energy status of the cell to the regulation of metabolism. 

Oxidative phosphorylation is regulated by cellular energy demands. The intracellular [ADP] and the mass-action ratio [ATP]/([ADP][Pi]) are measures of a cell s energy status. 

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