Phosphoenolpyruvate carboxylase
| Phosphoenolpyruvate carboxylase ( Zea mays ) | ||
|---|---|---|
| Mass / length primary structure | 970 amino acids | |
| Secondary to quaternary structure | Homotetramer | |
| Cofactor | Mg 2+ | |
| Identifier | ||
| Gene name (s) | pep (MaizeGDB) | |
| Enzyme classification | ||
| EC, category | 4.1.1.31 , carboxy lyase | |
| Substrate | H 2 O + phosphoenol pyruvate + CO 2 | |
| Products | Phosphate + oxaloacetate | |
| Occurrence | ||
| Parent taxon | Plants, various microorganisms | |
The enzyme phosphoenolpyruvate carboxylase ( PEPCase , PEPC ) irreversibly carboxylates phosphoenolpyruvate to oxaloacetate in plants, bacteria, algae and archaea. It is an essential switching point in the plant metabolism , which is why it can be used as a biomarker in plants . Various damage patterns (plant diseases with a causal relationship to insufficient supply) can be measured with considerable accuracy and the assessment of forest damage can be improved compared to the exclusive observation of the indicator plants .
discovery
In 1953, PEPC was isolated from spinach leaves for the first time and discovered in all plants, algae and bacteria that have already been examined, but not in animals or fungi. PEPC has also been identified in the genome of archaea, but this differs considerably from PEPC from bacteria and plants.
introduction
PEPC is an important enzyme of bacteria, archaea and higher plants, in particular for the reductive carbon dioxide fixation in plants with C 4 - or Crassulacean acid metabolism . In addition, it also plays a role in metabolism in the synthesis of carbon compounds during nitrogen fixation in legumes , regulation of the turgor , maintenance of the ion balance or regulation of the pH value.
Initially, it was assumed that the PEPC in eukaryotes originally came from protocyanobacteria according to the endosymbiont theory. However, PEPC from plants are more similar to those from γ-proteobacteria . Either the predecessor of the α-proteobacterium responsible for the mitochondrion or the predecessor of the cyanobacterium responsible for the chloroplast received the PEPC genes ( ppc ) via horizontal gene transfer (HGT) from that γ-proteobacterium. Alternatively, the PEPC genes from γ-proteobacteria were introduced relatively early in plant history via HGT.
properties
The enzyme is structurally a homotetramer with four active centers . It uses divalent magnesium (Mg 2+ ) as a cofactor . Three-dimensional structures are available from maize and Escherichia coli . The monomer consists of a β-barrel with eight β-sheets and 40 α-helices . The active center is located at the C-terminal area of the β-barrel. The active side is also similar to that of pyruvate kinase or pyruvate phosphate dikinase, even if they differ in their amino acid sequence.
There are differences in the PEPC of different organisms: In higher plants, each subunit of PEPC is up to 1,000 amino acids in size (up to 110 kDa ). The largest reported PEPC subunit comes from the alga Chlamydomonas reinhardtii (1,221 amino acids with 131 kDa). The subunits in Methanothermobacter thermautotrophicus , an Archeon , have a weight of 55 kDa , which corresponds to about half of the other variants. In general, archaeal PEPCs are much smaller than bacterial or vegetable PEPCs.
In plants, the ppc genes are highly conserved and usually have 10 to 11 exons and 9 to 10 introns . The arrangement of these exons and introns is also highly conserved. The number of ppc genes differs in different plant species, for example six in rice or four in thale cress . There are also different classes of PEPC. In the fully sequenced genomes of thale cress and rice, for example, there is a ppc gene that is more like bacterial PEPC.
The great importance of the PEPC is also evident in its spread. PEPC is found in Methanopyrus kandleri , an archaeon that usually lives in the hot puddles of volcanoes, which is unusual given the temperature dependence of PEPC.
reaction
PEPC is a lyase which is functionally specified as a C – C lyase or a carboxy lyase. It converts PEP and bicarbonate (HCO 3 - ) irreversibly in the following reaction:
 |
HCO 3 - P i |
|
| Phosphoenolpyruvate (PEP) | Oxaloacetate |
The Michaelis constant for the conversion of PEP (phosphoenolpyruvate) to oxaloacetate is 5 - 6 × 10 −4 mol / l for PEP and 3.1 × 10 −3 mol / l for HCO 3 -
mechanism
The reactants are transported into the reactive center of the enzyme. There the PEP is positioned relatively centrally, the Mg 2+ is brought as a cofactor close to the reaction center and then the hydrogen carbonate is moved close to the reaction site. For the exact reaction mechanism, however, it is not entirely clear whether PEP and magnesium bind to the enzyme separately or as a complex. It is discussed whether the energetic control of the reaction may even be influenced by the enzyme. After the reaction has ended, the products are discharged again, which is presumably facilitated or initiated by the enzyme.
regulation
PEPC is regulated by many factors. Thus, glucose-6-phosphate and other sugar phosphates (triose phosphates) activators photo synthetically produced, while aspartate , oxaloacetate and malate feedback inhibitors are. In non-photosynthetic isoenzymes, those inhibitors have a much stronger effect. In monocots , it is also inhibited by glycine . Glucose-6-phosphate increases the affinity of the enzyme for PEP and modulates that of its allosteric inhibitors.
The level of these metabolic activators and inhibitors is influenced by phosphorylation on an N-terminal serine residue of PEPC. Phosphorylation causes the enzyme to be switched on, as the sensitivity to the allosteric inhibitors malate and aspartate is reduced. The site of phosphorylation is highly conserved in eukaryotes, but was not found in bacteria. This phosphorylation is catalyzed by a Ca 2+ -independent, only 30 kDa serine / threonine kinase, the PEPC kinase (PEPC-K).
The PEPC found in archaea differs in that it cannot be regulated in the same way as in plants or bacteria, and the necessary allosteric binding sites are missing.
Use
Due to the genetic differences, there are minor deviations within botanically identical representatives of the same species , which, in addition to structural differences, is reflected in the measurable enzymatic activity and certain tolerances of trees and other plants to environmental toxins . With sufficiently accurate measurement methods, these differences are very reliable measures of the health of the plant.
ppc is a molecular marker that reacts to harmful effects from the environment within weeks to months . The enzyme reacts significantly to an (increased, and with slightly reduced accuracy, also decreased) nitrogen supply and to phosphate regulation ; Furthermore it can be shown that PEPC responds to ozone , the magnesium supply as well as heavy metals and pesticides . The response can be measured quantitatively by using corresponding clones of the plant to be examined as a reference. Even with artificial essays (model substances) one can achieve sufficient accuracy for practical damage assessment.
literature
- Udo Gowik and Peter Westhoff: C 4 -phosphoenolpyruvate carboxylase . In: Agepati S. Raghavendra, Rowan F. Sage (Ed.): C 4 photosynthesis and related CO 2 concentrating mechanisms . Springer, Dordrecht 2011, ISBN 978-90-481-9406-3 (series Advances in photosynthesis and respiration Volume 32).
- O'Leary B., Park, J. and Plaxton, WC. (2011): The remarkable diversity of plant PEPC (phosphoenolpyruvate carboxylase): recent insights into the physiological functions and post-translational controls of non-photosynthetic PEPCs. In: Biochem J . 436 (1); 15-34; PMID 21524275 ; doi : 10.1042 / BJ20110078
Individual evidence
- ↑ Swiss Institute of Bioinformatics (SIB): PROSITE documentation PDOC00330. Phosphoenolpyruvate carboxylase. Retrieved August 12, 2011 .