Many photosynthetic life forms (plants, algae,

phototroph Phototrophs () are organisms that carry out photon capture to produce complex organic compounds (e.g. carbohydrates) and acquire energy. They use the energy from light to carry out various cellular metabolic processes. It is a list of common m ...

ic and chemoautotrophic bacteria, and archaea) require a way to utilize carbon into their metabolic pathways. This usually occurs in pathways that fix carbon from carbon dioxide (CO₂). In the 3-hydroxypropionate bicycle, photosynthetic organisms like '' Chloroflexus aurantiacus,'' fix CO₂ and bicarbonate (HCO₃⁻ ) as part of their metabolic processes.

Pathway

The 3-hydroxypropionate bicycle, also known as the 3-hydroxypropionate pathway, is a process that allows some bacteria to generate 3-hydroxypropionate using

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

. It is divided into two parts or reactions. The overall reaction of the 3-hydroxypropionate pathway is . 3-Hydroxypropionatzyklus

Part I

In this pathway CO₂ is fixed (i.e. incorporated) by the action of two

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

acetyl-CoA carboxylase Acetyl-CoA carboxylase (ACC) is a biotin-dependent enzyme () that catalyzes the irreversible carboxylation of acetyl-CoA to produce malonyl-CoA through its two catalytic activities, biotin carboxylase (BC) and carboxyltransferase (CT). ACC ...

and

propionyl-CoA carboxylase Propionyl-CoA carboxylase (, PCC) catalyses the carboxylation reaction of propionyl-CoA in the mitochondrial matrix. PCC has been classified both as a ligase and a lyase. The enzyme is biotin-dependent. The product of the reaction is (S)-methylmal ...

. These enzymes generate

malonyl-CoA Malonyl-CoA is a coenzyme A derivative of malonic acid. Biosynthesis Malonyl-CoA cannot cross membranes and there is no known malonyl-CoA import mechanism. The biosynthesis therefore takes place locally: * cytosol: Malonyl-CoA is formed by c ...

and (''S'')-methylmalonyl-CoA, respectively.

Part II

Malonyl-CoA, in a series of reactions, is further split into

acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidation, o ...

and

glyoxylate Glyoxylic acid or oxoacetic acid is an organic compound. Together with acetic acid, glycolic acid, and oxalic acid, glyoxylic acid is one of the C2 carboxylic acids. It is a colourless solid that occurs naturally and is useful industrially. Str ...

Glyoxylate Glyoxylic acid or oxoacetic acid is an organic compound. Together with acetic acid, glycolic acid, and oxalic acid, glyoxylic acid is one of the C2 carboxylic acids. It is a colourless solid that occurs naturally and is useful industrially. Str ...

is incorporated into beta-methylmalyl-coA which is then split, again through a series of reactions, to release pyruvate as well as acetate, which is used to replenish the cycle. 3-Hydroxypropionatzyklus II

Prevalence

This pathway has been demonstrated in ''

Chloroflexus Chloroflexales is an order of bacteria in the class Chloroflexia. The clade is also known as filamentous anoxygenic phototrophic bacteria (FAP), as the order contains phototrophs that do not produce oxygen. These bacteria are facultative aerobic. ...

'', a nonsulfur photosynthetic bacterium; however, other studies suggest that 3-hydroxypropionate bicycle is used by several

chemotrophic A chemotroph is an organism that obtains energy by the oxidation of electron donors in their environments. These molecules can be organic (chemoorganotrophs) or inorganic (chemolithotrophs). The chemotroph designation is in contrast to phototroph ...

archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...

. T In ''

E. coli ''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escherichia'' that is commonly foun ...

'' 3-hydroxypropionate bicycle has been studied and found to be insensitive to oxygen. This means that within the pathways there is nothing that oxygen can affect because in either part of the pathway or the oxygen is used to drive the reaction forward.

References

External links