METABOLIC PATHWAYS (Unit 8 for JNTU, PBT)

Metabolic pathways are series of consecutive enzymatic reactions that produce specific products. Their reactants, intermediates, and products are referred to as metabolites. Since any organism utilizes many metabolites, it has many metabolic pathways which are interconnected.

Metabolic pathways can be divided into two categories : those invoving degradation (catabolism) and those involving biosynthesis (anabolism).

In catabolic pathways complex metabolites are exorgonically, broke doen to simple products.

Cataboism converts a large number of diverse substances (carbohydrates, lipids and protiens) to common intermediates.

These intermediates are then metabolized in a central oxidative pathway that terminates in a few end products.

Carbohydates, lipids and protiens are degraded to a common intermediate, acetyl- CoA, this is followed by oxidation of acetyl group to CO₂ and H₂O by a series of reactions.

Anabolism carries out the opposite process

Relatively few metabolites serve as starting materials for several varied bio-synthetic products.

Four important characteristics of metabolic pathways are:

1) Metabolic pathways are irreversible

2) Every metabolic pathway has a first commited step

3) All metbolic pathways are regulated

4) metabolic pathways in eukaryotic cells occur in specific cellular locations

METABOLIC PATHWAYS ARE IRREVERSABLE :

They are highly exorgonic (have large –ve free energy changes) so that their reactions go to completion. This characteristic provides the pathway with direction. Consequentially if two metabolites are metabolically interconvertible the pathway from the first to the second must differ from the second back to the first.

The reason for this difference is that if the route from the first metabolite to the second is exergonic, free energy must be sulloied in order to bring it back uphill. This requires a different pathway for atleast some of the reaction steps.

The existance of independent interconversion routes is an important property of metabolic pathways because it allows independent control of the rates of the two processes.

If metabolite 2 is required by the cell it is necessary to turn off the pathway from 2 to 1 while turning on the pathway from 1 to 2.

EVERY METABOLIC PATHWAY HAS A FIRST COMMITTED STEP :

Although metabolic pathways are irreversable, most of their component reaction function clise to equilibrium. Early in each pathway however, there is generally an irreversible (exergonic) reaction that commits the intermediate it produced to continue down the pathway.

ALL METABOLIC PATHWAYS ARE REGULATED :

In order to extert control on the flux of metabolites through a metabolic pathway it is necessary to regulate its rate limiting step.

Most metabolic pathways are therefore controlled by regulating the enzymes that catalyse their first committed steps. This is the most efficient way to exert control because it prevents the unnecessary synthesis of metabolites further along the pathway when they are not required.

METABOLIC PATHWAYS IN EUKARYOTIC CELLS OCCUR IN SPECIFIC CELLULAR LOCATION :

The synthesis of metabolites in specific membrane bounded subcellular compartments makes their transport between these compartments a vital component of eukaryotic metabolism.

Biological membranes are selectively permeable to metabolites because of the presence on membranes of specific transport proteins

For example, ATP is generated in the mitochondria but utilised in the cytosol. A transport protein in mitochondrial membrane facilitates the passage of ATP through the mitochondrial membrane.