Futile cycle

Substrate cycles are exceptional cases in metabolism. You seem pointless at first glance, as catabolic and anabolic reactions occur simultaneously. For this reason, they are also, especially in the English-language literature, as futile cycles (English futile = useless, useless) respectively.

Importance

Signal amplification

A substrate cycle is coupled to the consumption of energy, but without having a direct benefit. Opposing reactions such as the phosphorylation of fructose- 6-phosphate ( F -6P ) to fructose -1 ,6 -bisphosphate (F-1 ,6- bP; reaction of glycolysis ) or its reverse reaction ( reaction of gluconeogenesis ) therefore normally not run place simultaneously: both pathways are regulated separately.

Nevertheless, there are some tissues, for such a phenomenon is occupied. Reasons behind this are considered, for example, the gain of metabolic signals. In the above example, the phosphorylation of F6P to F-1 ,6- bP (100 molecules per time unit ) exceeds only slightly the opposite hydrolysis of F-1 ,6- bP to F6P. AMP by a lack of energy signal, the forward reaction, the enzyme ( phosphofructokinase ) and the reverse reaction ( fructose -1 ,6- bisphosphatase ) are adjusted in opposite directions ( increase to 120 molecules of PFK, lowering to 72 molecules of FBPase ). Thus, the metabolite flux increases from 10 to 50 units, he quintupled due to an approximately 20 % change in the reaction rate in both directions.

Generation of heat

" Destroyed " The substrate cycle shown here ( hydrolyzed) in the balance sheet ATP and taking heat Education

In the net balance does not create new products, but it is ATP hydrolyzed. The case inherent energy is released as heat:

This mechanism has been demonstrated for the flight muscle of the bumblebee. Bumblebees are after the end of winter, the first insects that can leave their caves. By the described substrate cycle produce dormant bumblebees heat they need for survival.

In humans, there is a rare inherited disorder, malignant hyperthermia. Here a substrate cycle is triggered by anesthetic gases in a process, to participate in the Calciuminonen. A regular involvement of substrate cycles in heat generation is controversial in mammals. However, the survival rate drops to rats at 5 ° C in the case of thyroid hypofunction in accordance with the fact that thyroid hormones stimulate the metabolism in most tissues.