Let us make an in-depth study of the role of phosphorus in energy trapping and transfer during cellular metabolism with diagram.
As early as 1905, phosphorus atom was found to be involved in a vital role in cellular metabolism, when Harden and Young discovered that alcoholic fermentation occurs only when inorganic phosphate (PO4– – -) is present.
This discovery was followed by eventual isolation of a large number of intermediary metabolites containing PO4 ℗ group attached to carbon atoms by phosphate ester linkages.
The importance of phosphorylated intermediates remained un-cleared for 25 years. Then in 1930’s, Meyerhof and Lipmann discovered that phosphate esters enable cells to trap much of the energy of the chemical bonds present in their food molecules.
During fermentation several intermediates are formed (e.g., D-1,3-diphosphoglyceric acid), that contain what are now popularly known as high-energy phosphate bonds. These high energy phosphate groups are usually transferred to acceptor molecules, where they can serve as sources of chemical energy for vital cellular process. The most important of the acceptor molecules is adenosine diphosphate (ADP) (Fig. 7.1) Addition of a high-energy ℗ group to ADP forms adenosine triphosphate (ATP).
ATP-The carrier of Free energy:
Karl Lohmann (1929) discovered ATP in muscle cells. Fritz Lipmann and Herman Kalckar (1941) were the first to recognize the role of ATP in energy metabolism. ATP or Adenosine triphosphate is a special carrier of free energy that occurs in the protoplasm in free state.
Chemically, ATP is a nucleotide consisting of adenosine (adenine + a ribose) and three phosphoryl groups (- PO32-) linked by a phosphoester bond followed by two phosphoanhydride bonds. Starting from ribose the phosphoryl groups are designated as a, p and y phosphates. The two phosphoanhydride bonds between α-β andβ -γ are called ‘high-energy’ bonds or ‘energy rich’ bonds which are symbolized by – P (squiggle-P).
Thus, ATP can be represented as:
AR – P ~ P ~ R The active form of ATP is usually a complex of ATP with Mg2+ or Mn2+.
When ATP is hydrolyzed a large amount of free energy is liberated which is utilized in various energy-requiring processes of the organisms. Hence, ATP is popularly known as energy currency or energy tablet of cells.
The hydrolysis of y-phosphate releases an energy equivalent of 8.15 kcal/mole or 34 kJ (older estimate 7.3 kcal/mole or 30.6kJ). Similarly the hydrolysis of β-phosphate releases 6.5 kcal/mole or 27.2 kJ. while the α-phosphate, attached to ribose, on hydrolysis yields 3.3 kcal or 13.8 kJ of energy per mole.
The Pi and PPi are known as ortliophosphate and pyrophosphate respectively An enzyme that catalyzes the transfer of a phosphoryl group from ATP to an acceptor is called kinase. For example, adenylate kinase (= myokinasc) catalyzes the inter-version of ATP, AMP and ADP.
Similarly hexokinase or glucokinase catalyzes the transfer of a phosphoryl group from ATP to only hexose or glucose. Chemical reactions which release free energy are called exergonic and when require free energy called endergonic. In biological systems, endergonic reactions are thermodynamically unfavorable which can be driven only when the reactions are coupled with exorgonic hydrolysis of ATP.
Regeneration of ATP:
In a cell ATP is consumed as soon as it is produced. The regeneration of ATP from ADP and Pi is called phosphorylation. Phosphorylation of ADP is an endergonic process which may occur in 3 different ways.
(i) Substrate-level phosphorylation:
ATP is formed by direct transfer of a phosphoryl group from a high energy compound to ADP without the involvement of ETS.
(ii) Oxidative phosphorylation:
It occurs only during aerobic respiration in mitochondria where ATP synthesis in linked to terminal oxidation of coenzymes like FADH2 and (NADH + H+) when FADH2 is oxidized through respiratory chain producing 2 ATP, and the oxidation of NADH + H+ produce three ATP.
It is the synthesis of ATP during the light driven electron transport in the thylakoid membrane of chloroplast. In higher plants it occurs by cyclic, non-cyclic and pseduocyclic processes. But in purple bacteria and green bacteria, due to absence of photosystem II, only cyclic photophosphorylation occurs.