In this article we will discuss about the fixation or assimilation of carbon dioxide in microorganisms.
Although most microorganisms can fix or assimilate carbon dioxide (CO2), only autotrophic ones use CO2 as their sole or principal carbon source.
The reduction or assimilation of CO2 takes place at the expense of much energy. Usually autotrophic microorganisms obtain the required energy by trapping light during photosynthesis (photoautotrophs), but some derive it from the oxidation of reduced inorganic electron donors (chemo- autotrophs).
Microorganisms can fix CO2 or convert this inorganic molecule to organic carbon and assimilate it in certain major ways, which are the Calvin cycle (also called Calvin-Benson cycle, or reductive pentose phosphate pathway), the reductive tricarboxylic acid cycle (also called reductive TCA cycle, or reverse citric acid cycle), the hydroxypropionate cycle or acetyl-CoA pathway.
Carbon dioxide (CO2) is incorporated by almost all microbial autotrophs using Calvin cycle, a special metabolic pathway. Although this cycle in most photosynthetic microorganisms, it is absent in archaea (archaebacteria), some obligately anaerobic bacteria, and some microaerophilic bacteria. These microorganisms usually use rest of the two above mentioned pathways.
The reductive tricaboxylic cycle is used by some archaea (archaebacteria), e.g., Thermoproteus, Sulfolobous and by bacteria such as Chlorobium, a green sulphur bacterium. Chloroflexus, a green non-sulphur photoautotroph uses the unique pathway of hydroxypropinonate.
Phototropic microorganisms (microalgae, cyanobacteria, purple and green bacteria), like plants, assimilate CO2 to produce carbohydrate principally through Calvin cycle (Fig 25.7). The latter is named for its discover Melvin Calvin and is also popular by the names Calvin-Bensen cycle or reductive pentose phosphate cycle.
The Calvin cycle requires NAD(P)H and ATP and two key enzymes, ribulose-1, 5-biphosphate carboxylase (ribulose bisphosphate carboxylase) and phosphoribulokinase. To understand the Calvin-cycle easily, it can be divided into three phases (carboxylation, reduction, and regeneration).
The Carboxylation Phase:
During this phase of CO2 fixation the enzyme ribulose-1, 5-biphosphate carboxylase or ribulose bisphosphate carboxylase (in short form called RUBISCO) catalyzes the incorporation of CO2 to ribulose-1, 5-biphosphate (RuBP) to generate two molecules of 3-phosphoglyceric acid (PGA).
The Reduction Phase:
3-phosphoglyccric acid (PGA) is reduced to glyceraldehyde 3-phosphate with the envolvement of two enzymes. Phosphoglycerate kinase enzyme reduces 3-phosphoglyceric acid into 1, 3-byphosphate glyceric acid which is then reduced to glyceraldehyde 3-phosphate by enzyme glyceraldehyde 3-phosphate dehydrogenase.
The Regeneration Phase:
During this phase the ribulose-1, 5-biphosphate (RuBP) is regenerated and carbohydrates such as fructose and glucose are produced. Glyceraldehyde 3-phosphate is converted to dihydroxyacetone phosphate (DHAP); this conversion is reversible.
Most of these two i.e., glyceraldehyde 3-phosphate and DHAP are used to regenerate ribulose-1, 5-biphosphate via various intermediate steps involving transketolase and transaldolase reactions, the remaining ones are used in the biosynthesis of carbohydrates.
The stoichiometry of the Calvin cycle can be represented in few words as 12 NADPH and 18ATP are required to synthesize 1 hexose molecule (glucose) from 6 molecules of CO2.
The overall equation can be summarized as under:
6CO2 + 18ATP + 12NADPH + 12H+ + 12H2O → 1 Hexose + 18ADP+ 18Pi + 12NADP+
Reductive or Reverse Tricarboxilic Acid Cycle (Reduced TCA Cycle):
The reductive tricarboxylic acid cycle (reduced TCA cycle, reduced Kreb’s cycle, or reduced citric acid cycle), also called reverse TCA cycle is used as alternative mechanism of CO2 fixation by phototrophite green sulphur bacteria (e.g., Chlorobium) and by some nonphoto-trophic archaebacteria (Thermoproteus, Sulfolobus and Aquifex).
In this cycle, the CO2 fixation takes place by a reversal of steps in the tricarboxylic acid cycle (a major pathway of respiration by which pyruvate is completely oxidised to CO2). In Chlorobium, there are two ferredoxin-linked enzymes that catalyse the reductive fixation of CO2into intermediates of the tricarboxylic acid cycle.
The two ferredoxin-linked reactions involve the carboxylation of succinyl-CoA to α-ketoglutarate and the carboxylation of acetyl-CoA to pyruvate (Fig. 25.8).
The reductive tricarboxylic acid cycle starts from oxaloacetate and each complete turn of the cycle results in three molecules of CO2 being incorporated and pyruvate as the product.
All reactions of the cycle are catalysed by enzymes of normal tricarboxylic acid cycle, but they work in reverse. One exception is citrate lyase, an ATP-dependent enzyme that cleaves citrate into acetyl-CoA and oxaloacetate in green sulphur bacteria.
Citrate lyase replaces citrate synthetase that produces citrate from oxaloacetate and acetyl-CoA in the normal TCA cycle. However, the acetyl-CoA of reduced TCA cycle produces pyruvate, which is converted to phosphoenolpyruvate that then results in triose-phosphate. The triose-phosphate converts into hexose- phosphate (glucose-phosphate), which is utilized in cell material.
Hydroxypropionate pathway (Fig. 25.9) is also a mechanism of autotrophic CO2 fixation unique to green non-sulphur bacteria (Chloroflexus). Choroflexus, an anoxigenic photoautotroph, uses either H2 or H2S as electron donors.
In hydroxypropionate pathway, two molecules of CO2 are reduced to glyoxylate. Acetyl-CoA is carboxylated to yield methylmanonyl-CoA. This intermediate is rearranged to yield acetyl-CoA and glycoxylate. The latter is converted to cell material.
Hydroxypropionate pathway has so far been confirmed only in Chloroflexus and appears to be of evolutionary significance. Chloroflexus is a “hybrid” photoautotroph in the sense that its photosynthetic mechanism shows features characteristic of both purple sulphur bacteria and green sulphur bacteria. Bacteriochlorophyll a located in the cytoplasmic membrane of cells of Chloroflexus is arranged to form a photosynthetic reaction centre structurally similar to those of purple bacteria.
Chloroflexus, on the other hand, contains bacteriochlorophyll c and chlorosomes (oblong bacteriochlorophyll-rich bodies bound by a thin, non-unit membrane lying attached to the cytoplasmic membrane in the periphery of the cell) like green sulphur bacteria.
It has thus been proposed that modern Choloroflexus may be a vestige of a very early phototrophic ancestor that perhaps first evolved a photosynthetic reaction centre and then received chlorosome-specific genes by lateral transfer.