Assimilation (biology)

Assimilation ( Latin assimilatio , adjustment, incorporation ' ) is the anabolic exchange of substances and energy in living beings , in which ingested, foreign inorganic and organic substances from the environment are converted into components of the organism, usually with the addition of energy. Depending on the approach, a distinction is made between carbon , nitrogen , sulfur , phosphate and mineral assimilation. Dissimilation refers to the reverse change of material and energy.

Carbon assimilation

Carbon assimilation (or C assimilation) is the most important assimilation process.

Principle of assimilation shown as a simplified scheme

Heterotrophic organisms (animals, fungi, protists, most bacteria) build up the body's own substances from organic substances that they take from the environment and thus serve as a source of carbon, among other things.

Autotrophic organisms (plants, some bacteria) produceenergy-rich, simple organic substancesfrom carbon dioxide (CO ₂ ) during carbon dioxide assimilation by adding energy and with the help of a reducing agent , which are converted into more complex molecules in the further metabolism.

Photoautotrophic organisms (plants, some bacteria) use light as an energy source. This form of assimilation is therefore called photosynthesis . An example of this is the formation of D - glucose (C ₆ H ₁₂ O ₆ ) from carbon dioxide (CO ₂ ) and water with the help of light energy:

6 H ₂ O + 6 CO ₂ → C ₆ H ₁₂ O ₆ + 6 O ₂

The required light energy is 2872 kJ / mol under standard thermodynamic conditions.

• All green plants and some bacteria (cyanobacteria) use water as a reducing agent or electron source during photosynthesis . Oxygen is also generated in the process, so that there is oxygenic photosynthesis.
• Other bacteria use hydrogen (H ₂ ), hydrogen sulfide (H ₂ S), sulfur or iron (II) ions as reducing agents. Since no oxygen is released here, this is called an anoxygenic photosynthesis.

Chemoautotrophic organisms (some bacteria) use chemical energy that they gain from exergonic chemical transformations. This form of assimilation is therefore called chemosynthesis or chemotrophy .

• They use inorganic substances as reducing agents, for example hydrogen (H ₂ ), hydrogen sulfide (H ₂ S), sulfur, iron (II) ions, ammonia (NH ₃ ) or nitrite . These reducing agents are oxidized at the same time to generate energy.

Phosphate assimilation in plants

The roots of the plants import phosphate ions (HPO ₄ ^2- ) from the floor over H ⁺ / PO ₄ ^3- - symporter in the cell membrane of rhizodermis cells. Phosphate is used u. a. as a substrate for the phosphorylation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP) in the cytosol ( glycolysis ), in the mitochondria ( citrate cycle ) and in the chloroplasts ( photosynthesis ). The added phosphate group can be used in further reactions for the synthesis of sugar phosphates, phospholipids or nucleotides .

Sulfur assimilation in plants

Sulfur is mainly absorbed in the form of sulfate (SO ₄ ²⁻ ) from weathered rock via H ⁺ / SO ₄ ²⁻ symporters (see phosphate assimilation) and especially in leaves over several steps with reduction in the synthesis of the amino acid cysteine used. Glutathione , ferredoxin , NADH , NADPH and O-acetylserine act as electron donors. In the plastids , cysteine ​​provides sulfur, bound in a sulfhydryl group, for the synthesis of methionine , another sulfur-containing amino acid. The sulfur of these two amino acids can subsequently be incorporated into proteins , acetyl-CoA or S-adenosylmethionine and transported in the form of glutathione via the phloem into the shoot, into the root tips and into fruits, where no sulfur assimilation takes place.

Nitrogen assimilation in plants

Plants and many bacteria use nitrogen from nitrate (NO ₃ ^- ) or ammonium (NH ₄ ⁺ ) to produce nitrogen-containing organic compounds.

Nitrate assimilation

The assimilation of nitrogen from nitrate occurs in plants, depending on the species, mainly in the roots or in the shoot . By reducing nitrate to nitrite and ammonium , it leads to the synthesis of asparagine and glutamine .

Like sulfate and phosphate, nitrate (NO ₃ ^- ) is absorbed into the roots through an H ⁺ symport. In the cytosol , nitrate is reduced by nitrate reductase to nitrite (NO ₂ ^- ). The reducing agent used is mainly NADH , as well as NADPH in non-green tissues. The dephosphorylation of a certain serine residue of nitrate reductase caused by light leads to the activation of this enzyme , while darkness leads to phosphorylation and thus enzyme inactivation. Therefore, nitrate is mainly assimilated during the day (during photosynthesis ). Nitrite is transported into the plastids , where it is reduced to ammonium by the nitrite reductase . The electrons required for the reduction are provided by ferredoxin , which receives electrons in roots from the NADPH formed in the oxidative pentose phosphate pathway . In green tissues, the electrons come from the photosynthetic electron transport chain. The expression of the nitrite reductase genes is increased by light and increased nitrate concentration, while asparagine and glutamine, as end products of nitrate assimilation, inhibit enzyme formation.

Summarized in formulas:

1st step (nitrate reductase): NO ₃ ^- + NADH + H ⁺ → NO ₂ ^- + NAD ⁺ + H ₂ O

2nd step (nitrite reductase): NO ₂ ^- + 6 Fd _red + 8 H ⁺ → NH ₄ ⁺ + 6 Fd _ox + 2 H ₂ O

Ammonium assimilation

In the plastids , the glutamate ammonium ligase catalyzes the incorporation of ammonium nitrogen in the form of an amido group into the amino acid glutamic acid (glutamate), which produces glutamine. In a second step, the glutamate synthase transfers this amido group as an amino group to 2-oxoglutarate, creating two molecules of glutamic acid:

1. Glutamate + NH ₄ ⁺ + ATP → Glutamine + ADP + Pi (glutamate ammonium ligase)
2. Glutamine + 2-oxoglutarate + e ^- → 2 glutamate (glutamate synthase)

NADH is used as an electron donor for glutamic acid synthesis in the root plastids and ferredoxin in the chloroplasts of the leaves. Ammonium can also be assimilated via glutamate dehydrogenase:

2-oxoglutarate + NH ₄ ⁺ + e ^- → glutamate + H ₂ O ( glutamate dehydrogenase )

The electron donor for this reaction is NADH in the mitochondria and NADPH in the chloroplasts.

The nitrogen built into glutamine and glutamate is used to synthesize other amino acids through transamination . These reactions, catalyzed by aminotransferases , correspond to the binding of the amino group of an amino acid to the carbonyl group of an intermediate from glycolysis ( 3-phosphoglycerate , phosphoenolpyruvate and pyruvate ) or from the citrate cycle ( α-ketoglutarate and oxaloacetate ).

An example of transamination reactions is provided by aspartate aminotransferase :

Glutamate + oxaloacetate → aspartate + 2-oxoglutarate

The aspartate (an amino acid) formed here is a substrate for asparagine synthetase :

Aspartate + glutamine + ATP → asparagine + glutamate + AMP + PP _i

As an amino acid, asparagine is not only a substrate for protein biosynthesis , but also serves to store and transport nitrogen based on its high N: C ratio.

The expression of the asparagine synthetase genes is reduced by light and carbohydrates. Therefore, the regulation of this enzyme is complementary to the regulation of the enzymes for glutamine and glutamate synthesis (glutamine or glutamate synthase). Consequently, if there is sufficient energy availability (lots of light, high carbohydrate concentrations), the synthesis of the relatively carbon-rich substances glutamine and glutamate is favored; When there is a shortage of energy (low light, low carbohydrate concentrations), the synthesis of low-carbon asparagine for the purpose of storing and transporting nitrogen predominates.

Nitrate and ammonium assimilation enable plants to produce all of the amino acids necessary for their metabolism . Humans and animals cannot synthesize certain amino acids themselves and have to obtain these as essential amino acids from their diet, which come directly or indirectly from plants. The essential amino acids include histidine , isoleucine , lysine , methionine , phenylalanine , threonine , tryptophan and valine .

As part of nitrogen assimilation, many legume species obtain ammonia (NH ₃ ) from the symbiosis with bacteria of the genus Rhizobium ("nodule bacteria"), which reduce elemental nitrogen (N ₂ ) to ammonia, while algae ferns ammonia from the symbiosis with N ₂ -reducing cyanobacteria .

literature

• Gerhard Richter: Metabolic Physiology of Plants. Stuttgart: Thieme 1998, ISBN 978-3-134420067 .

Individual evidence

1. ↑ ^{a b c d e} Horst Bannwarth, Bruno P. Kremer, Andreas Schulz: Basic knowledge of physics, chemistry and biochemistry , p. 381; ISBN 978-3642107665 .
2. ^ ^{A b} Peter Karlson, Detlef Doenecke, Jan Koolman, Georg Fuchs, Wolfgang Gerok: Karlsons Biochemie und Pathobiochemie , p. 443.
3. ^ Jan Koolman, Klaus-Heinrich Röhm: Pocket Atlas Biochemistry of Humans , p. 164.