Do plants have amino acids
Plants are able to synthesize all 20 amino acids required for the construction of proteins themselves. In addition, they make up a number of others, some of which we will cover in the following section.
In discussing glycolysis and the citric acid cycle, we had dealt with carbon-containing but nitrogen-free molecules. Synthetic chains of amino acids branch off from some of the intermediate products. Due to chemical similarities - and only a few starting substances - we can assign them to five groups (families):
- Glutamate family (based on alpha-Ketoglutarate)
- Aspartate family (based on oxaloacetate)
- Alanine-valine-leucine group (based on pyruvate)
- Serine-glycine group (based on 3-phosphoglycerate)
- Aromatic amino acids (based on phosphoenolpyruvate and erythrose-4-phosphate).
Where does the nitrogen come from? Plants absorb it in the form of nitrate and, to a lesser extent, in the form of ammonium ions. A few species, mainly legumes, live in symbiosis with nitrogen-fixing bacteria that are capable of reducing atmospheric nitrogen. The other plants are content with a nitrate reduction.
In a first step, nitrate is reduced to nitrite. The reducing substance required for this is NADH + H+, which occurs during glycolysis, the required enzyme is nitrate reductase. Glycolysis and nitrate reduction would thus be coupled with one another, which among other things also has the advantage that regenerated NAD+ required for the progress of glycolysis.
In a second step, nitrate has to be reduced further to form ammonium ions. Reduced ferredoxin, which in turn is obtained in the photosynthesis process, acts as an electron donor. The enzyme complex required for this is nitrite reductase, which is localized in photosynthesizing tissues in the chloroplasts.
The reaction takes place via an electron transport chain in which both NADP and FAD are involved. How the individual steps are related is still partly unclear. Free ammonium ions are toxic to every cell, so they have to be intercepted as quickly as possible. The quantitatively most important route for green plants is likely to be via the NADP+ - A dependent glutamate dehydrogenase reaction is in progress. This brings us to the first amino acid and introduces a discussion of the glutamate family. A second alternative should be mentioned beforehand: The glutamate itself can bind another ammonium ion and is thus converted into the amino acid glutamine. This reaction also takes place in the chloroplasts. Both reactions are ATP-dependent.
To understand the synthesis of amino acids, the influence of a group of enzymes, the transaminases, should be emphasized. These are able to transfer an amino group (predominantly) from glutamine to one alpha-To transfer keto acid. They thus play a sort of distributing role for amino groups. Germinating seeds that lack functional chloroplasts and therefore do not photosynthesize, obtain amino groups exclusively from the breakdown of existing reserve proteins and channel them into new amino acids, proteins and other nitrogen-containing compounds in the way outlined above.
These include the amino acids glutamic acid (glutamate), glutamine, proline and arginine. The synthesis of the first two can be done in different ways
Proline is produced by ring closure - via two intermediate stages - with the consumption of one ATP, NADH + H each+ and NADPH + H+. This indicates how energy-intensive biosyntheses are. This is not explicitly mentioned in the following, but one should keep in mind that every comparable biosynthesis chain consumes amounts of energy of this magnitude. Proline does not contain an amino group, so correctly it should not be called an amino acid but should be called an imino acid (its characteristic:> NH).
A conversion of proline into hydroxyproline only takes place at proline residues that are part of a polypeptide chain. Hydroxyproline is particularly common in a cell wall protein, extensin.
Arginine is a basic amino acid, i.e. it has a second amino group that is ionized under physiological conditions. Their biosynthesis runs through a total of eight intermediate stages. An important intermediate is ornithine, which can be formed in plant cells in two alternative ways. It serves as an acceptor for carbamyl phosphate. Due to its accumulation, with simultaneous elimination of phosphate, the molecular chain of the ornithine is lengthened, and citrulline is formed.
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