Photosynthesis and Amino acid production in Plants

 Photosynthesis and Amino acid production in Plants

 

Green plants absorb and transform solar energy into chemical energy through the process of photosynthesis. During this, CO2 is reduced to simple carbohydrates and O2 is liberated into the atmosphere. This ability to capture the energy of sunlight and utilize it in manufacturing food is a unique property of green plants. Photosynthesis is the only process in the biosphere by which organic compounds are synthesized from inorganic raw materials. It results in the production of sugars (Carbohydrates).  Sugars not only serve as a source of energy but also as starting materials for other important synthetic reactions. (Amino acids, proteins, fatty acids, and lipids).  

Energy + 6 CO2 + 12 H2O    →    C6 H12 O6 + 6 O2 + 12 H2O

Chloroplasts

 

Each chloroplast is surrounded by a double membrane system or envelope that controls the passage of molecules in and out of it. The interior of the chloroplast is filled with a gel-like enzyme-rich material called the stroma. The stroma contains enzymes that convert CO2 into carbohydrates. Each chloroplast is surrounded by a double membrane system or envelope that controls the passage of molecules in and out of it. The interior of the chloroplast is filled with a gel-like enzyme-rich material called the stroma. The stroma contains enzymes that convert CO2 into carbohydrates.

Light reaction in Photosynthesis

 

The chlorophyll and other pigment molecules are embedded in the thylakoid membrane of the chloroplast.  They are organized into discrete units called photosystems. These pigment molecules are organized into two closely linked compounds, a reaction center complex and an antenna protein complex.  All the pigment molecules within a photosystem are capable of absorbing light energy but, only chlorophyll –a molecule of the reaction center can use the light energy in the photochemical reaction. In addition to chlorophylls, carotenoid pigments are also located in each antenna complex. The two photosystems are referred to as photosystem I and photosystem II.

In the photosystem, I chlorophyll –molecules of the reaction center are a form of chlorophyll known as P700. These special chlorophyll molecules have an absorption maximum of 700nm.  Chlorophyll molecules of photosystem II reaction center absorb maximally at 680nm and are named P680. The two photosystems are linked together by an electron transport chain. The arrangement of photosystems and electron carriers in the thylakoid membrane is diagrammatically shown below.

Photophosphorylation

A part of the energy that is released during the transport of electrons through the electron carriers is harvested to synthesize ATP. This synthesis of ATP during the light reaction of photosynthesis is called Photophosphorylation. There are two pathways by which the energy from electrons is harvested.

Dark reaction/biochemical reaction

 

The ATP and NADPH now move out into the stroma of the chloroplast and reduction of CO2 to carbohydrates occurs. Light is not directly involved in these biochemical reactions of photosynthesis. This series of reactions by which atmospheric CO2 is fixed and reduced to form carbohydrates were elucidated by Melvin Calvin, an American biochemist, and his associates.

First stable compound: Phosphoglyceric acid (PGA) 

• CO2 acceptor : Ribulose- 1,5-bisphosphate (RuBP) 

• PGA is not a sugar. It must be reduced into triose-p 

• RuBP is regenerated from triose-p

Alpha-ketoglutarate (AKG)

 

“Alpha-ketoglutarate (AKG) is a key molecule in the Krebs cycle determining the overall rate of the citric acid cycle of the organism. It is a nitrogen scavenger and a source of glutamate and glutamine that stimulates protein synthesis and inhibits protein degradation in muscles. AKG as a precursor of glutamate and glutamine is a central metabolic fuel for cells of the gastrointestinal tract as well. AKG can decrease protein catabolism and increase protein synthesis to enhance bone tissue formation in the skeletal muscles and can be used in clinical applications. In addition to these health benefits, a recent study has shown that AKG can extend the lifespan of adult Caenorhabditis elegans by inhibiting ATP synthase and TOR. AKG not only extends lifespan but also delays age-related disease. In this review, we will summarize the advances in the AKG research field, in the context of its physiological functions and applications.”

Amino acid synthesis and the connection of the Calvin cycle in plants

Nitrogen is one of the most prevalent elements within living organisms.  It is found in many essential compounds like proteins, nucleic acids, chlorophyll, many vitamins, and certain growth substances. So, nitrogen is involved in many biochemical reactions that sustain life.

NH4 + and NH3 (ammonia nitrogen) are toxic to plant cells if present in high concentrations.  So these ions do not accumulate in plant cells. They are rapidly incorporated into organic compounds.

Assimilation of ammonia nitrogen into carbon compounds

Two pathways

First pathway

Involves reductive amination of α-ketoglutarate (from Krebs cycle) to produce glutamate

α-ketoglutarate + NADH + NH¬3   →     Glutamate + NAD+ + H2O

The reaction is catalyzed by Glutamate dehydrogenase (found in chloroplast, and mitochondria)

Second pathway

Involves a reaction with glutamate to form its amide (Glutamine) 

Glutamate + NH3 + ATP (energy)  → Glutamine + ADP + Pi 

The reaction is catalyzed by the glutamine synthetase enzyme

Glutamine + α-ketoglutarate  → 2 Glutamate

 

Once assimilated into glutamate, nitrogen can be incorporated into other amino acids through transamination reactions. These reactions are catalyzed by aminotransferases enzyme. Then form amino acids like alanine, aspartic acid, glycine, and serine

Glutamate + Oxaoacetate  → Aspartate + α-ketoglutarate

Reductive amination

α-ketoglutaric acid + NH3→ Gutamic acid + H2O

Plants can produce all 20 amino acids that require for protein synthesis on their own by nitrogen metabolism. Animals have to get most of the amino acids from their diet.

Enzymes regulate all types of biochemical reactions in the cell and all enzymes are mainly proteins.  Proteins are polypeptide chains formed by bonding a large number of amino acids.  So amino acid synthesis is an important step in the incorporation of absorbed nitrogen into proteins by plants.

Written by: Rashmi Premathilake