Special Nitrogen Metabolism
Md. Abdur Rakib, PhD Associate Professor
Dept. of Biochemistry and Molecular Biology University of Rajshahi
Rajshahi-6205
Introduction Introduction
• Nitrogen is the major limiting nutrient for most plants.
• The main exogenous sources are atmospheric N2 or soil- derived nitrate (from fertilizers, manure, degradation of organic matter) which enter plant metabolism by nitrogen fixation and nitrate reduction
• The resulting NH4+ is used to build up the various amino acids.
• Twenty amino acids constitute the basic building units for all cellular proteins and also provide the nitrogen and/or carbon skeletons for various metabolites of low molecular weight.
-Plants produce a wide variety of secondary metabolites
-Over 13 000 nitrogen-containing secondary metabolites have been described so far.
- Most nitrogen-containing secondary compounds are derived from amino acids which donate the carbon skeleton and/or the nitrogen.
Introduction
Introduction
Non-protein amino acids Non-protein amino acids
Proteins of all organisms are based on the 20 common L-amino acids. More than 900 secondary compounds have been identified in plants, which are classified as amino acids, because they carry a carboxyl and an amino or imino group.
These amino acids are not building blocks of proteins in plants (therefore called 'non-protein amino acids') but are often antinutrients or antimetabolites, i.e. they may interfere with the metabolism of micro-organisms or
herbivores.
Structures of nonprotein amino acids and their functional equivalents
Structures of nonprotein amino acids and their functional
equivalents
Biogenetic pathways of a few non-protein amino acids
which derive from L-cysteine and L-asparagine Biogenetic pathways of a few non-protein amino
acids
which derive from L-cysteine and L-asparagine
Allelochemical effects and ecological role of non- protein amino acids
Allelochemical effects and ecological role of non- protein amino acids
If non-protein amino acids are taken up by herbivores, micro-organisms or other plants, they may interfere with their metabolism in the following ways:
1.They can be accepted in ribosomal protein biosynthesis in place of the normal amino acid leading to defective
proteins (e.g. canavanine, azetidine-2-carboxylic acid, 2- amino-4methylhexenoic acid, etc.)
2. They may inhibit the activation of aminoacyltRNA synthetases or other steps of protein biosynthesis.
3. They may competitively inhibit uptake systems for amino
acids (e.g. azetidine-2-carboxylic acid, 3,4-dehydroproline).
4. There may be inhibition of amino acid biosynthesis by substrate competition or by mimicking end-product mediated feedback inhibition of earlier key enzymes in the pathway (e.g. azaserine, albizzine, 5-aminoethyl cysteine).
5. They may affect other targets, such as DNA-, RNA- related processes (canavanine, mimosine), receptors of neurotransmitters, inhibit collagen biosynthesis
(mimosine), or /beta-oxidation of lipids (L-hypoglycine)
Allelochemical effects and ecological role of non- protein amino acids
Allelochemical effects and ecological role of non-
protein amino acids
Amines Amines
Depending on the degree of substitution, it is possible to distinguish primary, secondary, tertiary or
quaternary amines and as chemical classes: aliphatic mono- and diamines, polyamines, aromatic amines and amine conjugates.
Amines are widely distributed secondary metabolites in plants and occur as independent products in their own right or as intermediates in the biosynthesis of
alkaloids.
Some Aliphatic monoamines
Some Aliphatic monoamines
Function of Aliphatic monoamines Function of Aliphatic monoamines
Aliphatic monoamines mimick the smell of rotten meat,
carrion-feeding insects are attracted which visit the flowers or fruiting bodies of fungi and carry away pollen or spores and thus contribute to pollination or spore distribution.
A highly evolved system can be found in members of the Araceae. In (Fig. 12.5), flower morphology, primary and
secondary metabolism are highly co-ordinated to achieve a successful fertilization.
The spadix is rich in starch. When the flower is mature, starch is broken down rapidly, resulting in pronounced
thermogenesis.
As a result, the club of the spadix becomes quite warm. This is important for the dissipation of aliphatic monoamines.
The smell of the amines attracts pollinating insects.
Glucosinolates Glucosinolates
Glucosinolates resemble cyanogens in many respects, but contain sulfur as an additional atom.
They can be classified as thioglucosides.
When hydrolyzed, glucosinolates liberate D-glucose, sulfate and an unstable aglycone, which may form an isothiocyanate (common name 'mustard oil') as the main product under
certain conditions, or a thiocyanate, a nitrile or cyano- epithioalkane.
Isothiocyanates are responsible for the distinctive, pungent flavor and odor of mustard and horseradish.
Structure and occurrence of glucosinolates
Structure and occurrence of glucosinolates
Glucosinate Biosynthesis Glucosinate Biosynthesis
thiohydroximate glucosyltransferase.
Phosphoadenosinephosphosulfate (PAPS)
Auxin, Cytokinin and Ethylene Auxin, Cytokinin and Ethylene
Growth and development (germination, formation of roots and shoots, senescence) of plants is governed by special growth factors or phytohormones.
From them auxins, cytokinins and ethylene nitrogen containing.
Phytohormones are synthesized at the place where
they are needed.
Auxin Biosynthesis Auxin Biosynthesis
The main active auxin is indole-3-acetic acid (lAA) which is synthesized in the shoot tips of growing plants, e.g. of a seedling. The biosynthesis starts with the amino acid tryptophan and can follow three different routes, i.e. via indole-3-acetamide, tryptamine or indolepyruvic acid.
Function of Auxin Function of Auxin
Auxin induces the elongation of cells which lie underneath the tips.
In addition, auxin mediates the various tropisms in plants (i.e.
bending of climbers, movement of leaves).
Auxin seems to regulate gene expression in various growth and developmental processes, i.e. some specific mRNAs are 500- fold enhanced after IAA treatment.
Receptor proteins which bind auxin with high affinity have been described from the cytoplasmic membrane and characterized genetically.
Cytokinin Biosynthesis
Cytokinin Biosynthesis
The cytokinin zeatin which is characterized by a hydroxylated prenyl substituent has been isolated from maize N6-
dimethylallyladenine is closely related and derives from adenosine monophosphate condensed with dimethylallyl diphosphate.
Cytokinin Function Cytokinin
Function
Cytokinins are phytohormones which stimulate cell
division (activation of replication and transcription) and delay senescence.
In buds they function as an antagonist of auxin; for
example, the local application of cytokinins to soybean flowers prevents their premature abortion and
increases seed yield.
It has been assumed that cytokinins bind to membrane
receptors and elicit a series of cellular signals.
Ethylene: Biosynthesis Ethylene: Biosynthesis
Ethylene is derived from the amino acid L-methionine via
SAM, and 1-amino cyclopropane-1-carboxylic acid (ACC). The latter step is catalyzed by 1-aminocyclopropane-l-carboxylic acid (ACC)-synthase, an enzyme whose gene has already been isolated and cloned. 1-aminocyclopropane-l-carboxylic acid (ACC) is oxidized by ACC oxidase to ethylene and
cyanoformic acid (which decarboxylates spontaneously to HCN and CO2) (Luckner, 1990).
Ethylene: Function Ethylene: Function
Ethylene is a small volatile phytohormone which can easily diffuse through biomembranes and may influence their
integrity.
Ethylene affects several developmental processes such as the ripening of fruits or the defoliation of leaves.
Because its synthesis is generally enhanced in stressed tissues (i.e. by wounding, infection, water stress, etc.) ethylene may activate the biosynthesis of some defense chemicals.
Ethylene stimulates the flow of latex, which can be considered as a sort of defense reaction.
It has been suggested that aerial communication between plants might be mediated by ethylene.
In molds, ethylene is used to allow hyphae to pass obstacles without touching .
2-Chloroethylphosphonic acid (= ethephon, ethrel)
decomposes to ethylene in neutral or alkaline solution and is used commercially to exercise ethylene effects (e.g. in fruit ripening).
