Amidation

 

Amides 

These are derivatives of amino acids in which __ OH of the carboxylic group is replaced by another amino group _ NH2. The process of formation of amides is called amidation. The general  formula of amides is R.CH ( NH2). CO.NH2. Amides contain more nitrogen   than amino acids and are structural part of most proteins

(i)  Direct amidation : Ammonia is taken up directly  by an amino acid to produce an amide. Direct amidation is  known to occur in higher plants only in glutamic acid. It takes place in the presence  of  enzyme  glutamine  synthetase. A similar enzyme, asparagine synthetase is present in yeast for synthesis of asparagine. 


(ii) Transmidation : Amides other than glutamine are supposed to be formed by the transfer of amino group from an amide to an amino acid. The enzyme  used is transamidase. 

Since  amides contain more nitrogen than the amino acids, they are transported to other parts of the plant via xylem vessels. In addition  along with the transpiration  stream, the nodules of some plants (e.g., soya bean) export the fixed nitrogen as ureides. These compounds also have a particularly  high nitrogen to carbon ratio. 


What  is the process of  amidation reactions? 



Amidation is a chemical reaction that involves the conversion of a carboxylic acid or its derivative (such as an ester or an acid chloride) into an amide. The process of amidation typically involves the reaction of the carboxylic acid derivative with an amine compound.


The reaction begins with the nucleophilic attack of the amine on the carbonyl carbon of the carboxylic acid derivative. This forms a tetrahedral intermediate, which then undergoes rearrangement to create an acyl intermediate. The acyl intermediate is subsequently attacked by another amine molecule to yield the desired amide product.


The mechanism of amidation can vary depending on the specific reaction conditions and the reagents used. For example, amidation can be achieved using a variety of catalysts or reagents, such as acid chlorides, acyl isocyanates, or activated esters, which react with primary or secondary amines. In some cases, activated carboxylic acids can directly react with amines to form amides.


Amidation reactions are commonly used in organic synthesis and pharmaceutical research, as amides are important functional groups found in many biologically active compounds.

Amides structure  Examples 

The two most important  amides _ asparagine and glutamine _ found in plants are a structural part of proteins. They are formed from two amino acids,  namely aspartic acid and  glutamic acid, respectively  , by addition  of another amino  group to each. The hydroxyl part of the acid is replaced by another NH2- radicle. Since amides contain more nitrogen than the amino acids, they are transported to other parts of the plant via xylem vessels. In additional  along with the  transpiration  stream the nodules  of some plants (e.g,. Soyabean) export the fixed nitrogen as ureides. These compounds also have a particularly  high nitrogen to carbon ration. 


Mineral Fertility :

Minerals get depleted  in the soil due to several reasons: 
(i) Overcropping which withdraws minerals from  the top layers of the soil.

(ii) Use of high yielding  varieties,  require higher quantity  of minerals nutrients

(iii) Non_ rotation of crops.

(iv) Leaching or washing down of minerals alongwith gravitational  water 

(v) Precipitation of minerals due to change in soil pH. 

Synthesis  of Amino Acids

It occurs mostly in the roots and leaves where nitrates are reduced. The cells  of these regions have to provide the necessary  organic acids which are mostly formed in Krebs cycle. As Krebs cycle occurs in mitochondria, the sites of amino acid synthesis  are also assumed to be the mitochondria. Amino acids  are formed  by the following  methods: 


(i) Reductive  amination: In the presence of dehydrogenase  and a reducing  power ( either NADH2 or NADPH2) , ammonia can combine directly  with a keyo acid to produce  an amino acid. The substrate  is generally  a_ ketoglutaric acid, an intermediate  of Krebs cycle. It gives rise to glutamic acid. 


Besides æ_ ketoglutaric  acid other  organic acids which undergo reductive animation  are oxaloacetate and phosphoenol pyruvate.  

(ii)  Transamination : It involves  the transfer of amino group from one amino  acid  to the keto  group of keto acid.  The reaction is carried out in the presence of an enzyme named transaminase. It requires coenzy pyridoxal phosphate, a derivative  of pyridoxine  or vitamin  B6. Teansamination reactions are reversible. Glutamic  acid is the main amino acid  from which other 17 amino acids are formed through transamination. 

(iii) Transformation: A number of amino acids  are produced  from others by chemical transformation  through  oxidation, reduction, condensation  etc. 

(a) Oxidation: e.g., formation of hydroxyproline from proline. 

(b) Reduction : e.g., aspartic acid loses oxygen  to form homoserine. 

(c) Condensation : e.g., two molecules  of glycine condense to form serine with the release  of a CO2 molecule . 

(iv) Catalytic amidation: Ammonia combine with catalytic  amounts  of glutamic acid in the presence  of ATP and enzyme glutamine  synthetase. It produces the amide glutamine. 

 Glutamine  reacts with æ_ Ketoglutaric  acid  in the presence of NADH or NADPH and enzyme glutamic acid synthetase to form two molecules  of glutamic acid.  

Restoration of Mineral Fertility  

It is done through  two methods, natural and artificial. 

(a) Natural Replenishment  : 

(i) Recovery of minerals from decomposition   of fallen leaves, dead roots, dead aninals and animal excreta.

(ii) Slow process of weathering  of bed rock.

(iii) Burrowing habit of some animals  like earthworms which bring subsoil over the surface of top soil. 

(iv)  Biological  nitrogen fixation. 

(v) Passing down of nitrogen, sulphur and other  salts from atmosphere  to soil through  rain. 

Artificial  Replenishment: 

It  is carried out by human efforts. 

(i) Addition of farmyard manure. Manure contains humus or dark brown amorphous substance  formed by degradation  of cellulose and lignin. Humus forms colloidal particles  in soil for increased  hydration, aeration, loosening  the soil, slow release  of minerals  and organic substances. 


(ii) Green manuring. A young green leguminous  crop is ploughed back into the soil for providing  humus, decreasing  soil alkalinity  and increasing  nitrogen  supply.


(iii) Crop rotation. Usually  legume crops are rotated with non_ legume crops. As the different crops withdraw minerals from different  depths, the soil does not become deficient  in minerals rapidly. 

(iv) Addition of fertilizaters. 


Fertilizaters 

The most common minerals elements  which become deficiency  in agriculture  soils are  nitrogen, phosphorus  and potassium  ( NPK). They are called critical  elements. Majority  of the fertilizers, therefore,  contain all the three critical elements. Such fertilizers  are called complete fertilizers. Common fertilizers consist  of chemicals, either singly ( simple  or straight fertilizer) or in various combinations ( compound or multinutrient fertilizer), like urea, nitrate of soda, ammonium sulphate,  ammonium, nitrate, ammonium, chloride, calcium  ammonium nitrate, superphosphate, calcium magnesium phosphate, rock phosphate, bone meal, etc. Ammonium  is a better source  of nitrogen for areas with high rate of leaching  and photosynthesis. The common complete or NPK fertilers contain nitrophosphate with potash. The percentage  of nitrogen, phosphorus and potassium  by weight is mentioned, such as 15 _ 15_ 15, 17 _ 18_ 19, etc. 


Utility  of micronutrient  in fertilizer  depends upon the deficiency  in soil and pH of  the soil. For example,  in alkaline  soil Cu, Zn, Mn and Fe become easily precipitated.  It is preferable  that  deficient micro_ elements  and costlier macronutrients be sprayed over the plants. The practice  is called foliar nutrition or foliar fertilizers. 














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