3-Hydroxypropionate pathway

The 3- Hydroxypropionatzyklus, also 3-Hydroxypropionat/Malyl-CoA-Zyklus, is a pathway by which certain microorganisms carbon dioxide ( CO2) can fix. These two molecules of CO2 in the form of bicarbonate (HCO3-) be made to a molecule glyoxylate under ATP and NADPH consumption. Subsequently, the resulting glyoxylate is converted by a subsequent cycle, while fixing a further molecule of bicarbonate into pyruvate. The 3- Hydroxypropionatzyklus was first discovered in 1986 in the phototrophic bacterium Chloroflexus aurantiacus.

Occurrence

The 3- Hydroxypropionatzyklus has been detected only in the microaerophilic bacterium Chloroflexus aurantiacus Green non sulfur. Although closely related bacteria ( Chloroflexus aggregans, Roseiflexus spp.) Have the genes for this pathway, not autotrophic growth was detected in them. Another relative, Oscillochloris sp., On the other hand uses the Calvin cycle and not the 3- Hydroxypropionatzyklus CO2 fixation.

Biochemistry

3- Hydroxypropionatzyklus consists of two interconnected cycles. In the first cycle, glyoxylate is formed from two molecules of bicarbonate. The glyoxylate is then reacted in a second cycle, to pyruvate, for the regeneration of the second cycle, another molecule bicarbonate used. Three molecules of bicarbonate So it will be up to one molecule of pyruvate.

Formation of glyoxylate

Starting from acetyl-CoA cycle starts with the condensation of one molecule of bicarbonate, in which a molecule of ATP is consumed. This reaction is catalyzed by a biotin -containing acetyl -CoA carboxylase, malonyl-CoA is produced. This is reduced to 3- hydroxypropionate by a malonyl-CoA - reductase with NADPH consumption. 3- hydroxypropionate is the eponymous component of this cycle. Propionyl -CoA synthase catalyzes the formation of propionyl-CoA, NADPH is required during this step as a reducing agent again.

The second step is carried out at the Bicarbonatassimilation the conversion of propionyl -CoA to methylmalonyl -CoA under the consumption of ATP, which provides a propionyl -CoA carboxylase. Methylmalonyl -CoA is rearranged via succinyl -CoA and L- malate to L- malyl -CoA. In this reaction sequence, an epimerase, a mutase, a transferase, a dehydrogenase and fumarase ( fumarate hydratase ) are involved, which also FADH2 is formed.

A malyl -CoA lyase enzymatic part of the trifunctional Malyl-CoA/β-Methylmalyl-CoA/Citratmalyl-CoA-Lyase ( see also below), finally splitting L- malyl -CoA to glyoxylate and acetyl -CoA, resulting in the circuit closes again.

The balance sheet for the formation of glyoxylate is:

Formation of pyruvate

In the following second cycle, the glyoxylate structured is converted to pyruvate. Here, each condense one molecule of glyoxylate and propionyl- CoA to β - methylmalyl -CoA, which is catalyzed by the participated in the first cycle β - methylmalyl - CoA lyase ( Malyl-CoA/β-Methylmalyl-CoA/Citratmalyl-CoA-Lyase ). β - methylmalyl -CoA is converted by a mesaconyl -CoA hydratase to mesaconyl -C1 -CoA. Mesaconyl -C1 -CoA corresponds to a chemical point of 2- Methylfumaryl -CoA. In C. aurantiacus further reaction sequence was decoded recently. Mesaconyl -C1 -CoA is transformed to mesaconyl - C4 -CoA. This intramolecular transesterification of Coenzyme A catalyzed by transferase. Mesaconyl - C4 -CoA is then converted to (3S)- Citratmalyl - CoA, a hydratase catalyzes. This will eventually cleaved to acetyl -CoA and pyruvate - by Citratmalyl -CoA lyase enzymatic part of the Malyl-CoA/β-Methylmalyl-CoA/Citratmalyl-CoA-Lyase. Acetyl- CoA is constructed as in the first part of the 3- Hydroxypropionatzyklus to propionyl-CoA. Pyruvate is then finally metabolized via a triose on.

The balance sheet for the formation of pyruvate from glyoxylate is:

The overall balance for the formation of a molecule of pyruvate runs as a result:

Biological Significance

The fixation of three molecules of bicarbonate into pyruvate is an overall very energy- consuming process. The setting up of a molecule of glyceraldehyde -3-phosphate costs 10 ATP equivalents ( the AMP formed is counted twice ). The process can run well under aerobic or microaerophilic conditions, since none of its enzymes involved per se are sensitive to oxygen. In addition to the fixation of bicarbonate but may also be an intermediate in the cycle are assimilated (acetic acid, propionic acid, C 4- dicarboxylic acids ). These are excreted as fermentation products of associated microorganisms.

A special feature of the cycle is that bi-and trifunctional enzymes are involved. Thus, the malonyl -CoA reductase catalyzes two reaction steps Malyl-CoA/β-Methylmalyl-CoA/Citratmalyl-CoA-Lyase and propionyl -CoA synthase can catalyze three reactions. The entire cycle is carried out in 19 reactions, but only 13 enzymes are involved in the reactions.

In a similar form, the 3-Hydroxypropionat/4-Hydroxybutyratzyklus, fixing bicarbonate was detected in thermoacidophilic Archaea of the genera Metallosphaera, Acidianus and Sulfolobus.

The use of bicarbonate in place of carbon dioxide can be explained as C. aurantiacus waters is growing at a slightly alkaline environment. Under these conditions, the concentration of bicarbonate is greater than that of carbon dioxide.