Biochemistry Lecture No. 15: Metabolism & Enzyme Regulation
Thursday October 11 , 2012
-Metabolism is the sum of all reactions that go in the cell (half of them being catabolic, half anabolic).
-Metabolic pathways are a series of reactions coupled together that form a new product. Flux is the rate
of conversion from the reactants to the final product of a metabolic pathway. This conversion is efficient
when the net ΔG for the whole pathway is negative. Generally, a metabolic pathway consists of mostly
reactions at equilibrium (ΔG ≈ 0), but some are reactions with great spontaneity (ΔG << 0) and are
-With equilibrium reactions, where ΔG ≈ 0, changes in concentration heavily influence the extent of the
reaction by way of le Chatelier’s Principle. Enzymes used here can catalyze both the forward and reverse
reactions and are considered reversible. With highly spontaneous reactions, where ΔG << 0, adding
products has little to no effect on the equilibrium of the reaction (which is hardly observed). There is
practically no reverse reaction observable as the E Aor the reverse reaction is much harder to overcome.
-As irreversible reactions drive the overall process of metabolic pathways, the cell must increase the
activity of the enzyme(s) catalyzing only those “steep” (energetically favourable) reactions. Thus,
irreversible reactions are the target of regulation.
Methods For Controlling The Rate Of Reaction:
-Rates of reaction in metabolic pathways can be manipulated in several ways: by changing the amount
of the enzyme expressed (during transcription, translation, or degradation), changing the concentration
of the substrate or product (using le Chatelier’s Principle), or by changing enzyme activity (through
allosteric regulation, covalent modification, or association with a regulatory protein). Changing enzyme
abundance often takes very long to take effect and changing substrate or product concentration is often
impractical for the cell, while changing enzyme activity usually produces the quickest results.
-In allosteric enzymes, the shape of the active site (where the substrates binds) is changed by the
binding of an effector at a different site known as the allosteric site. Allosteric enzymes are usually
multimeric and contain more than subunit.
Multimeric Allosteric Enzymes:
- Allosteric enzymes are usually multimeric and contain more than subunit. There is a distinction
between these subunits as regulatory subunits have binding sites that activate or inhibit its protein’s
activity (through the allosteric site), while catalytic subunits help the proteins catalyze a ch