CONCEPTS OF METABOLISM

A. Metabolic pathways

B. Control of metabolism

C. Role of free energy

A. Metabolic pathways

Chemical reactions in a cell (e.g., 2000-4000 in asimple bacterium)

! Synthesis of fatty acids, amino acids, sugars; lipids,proteins, polysaccharides

! Breakdown of lipids, proteins, polysaccharides: foodto be used for resynthesis; toxic wastes

! Other reactions: harvesting energy; repair;synthesis of “secondary compounds” for signalling

A metabolic map

A simplifiedmetabolic map,as shown inyour textbook(Fig. 6.15)

Each dot in the diagram represents a compound. Each line represents a simple chemical reaction. The reactions are arranged in series, called pathways. A ---> B ---> C ---> D ---> F

Each arrow represents one chemical reactioncatalyzed by one enzyme.

Sometimes pathways are more complex:

Sometimes pathways are more complex:

B. Control of metabolism

Basic principle: control metabolism by controlling the amount or activity of appropriate enzymes.

1. Control amount of enzyme protein

2. Control activity of existing enzymes

(a) Allosteric control

! "Allosteric" = other shape! Reversible change in enzyme's shape! “Inhibition” or “activation”! Allosteric enzymes have at least two

polypeptide chains (catalytic subunit andregulatory subunit),

P

P

Starch(n) + P Starch(n-1) + glucose-P

Starch phosphorylase(inactive form)

Starch phosphorylase(active form)

PProtein phosphorylase

Protein kinase

ATP

ADP

(b) Phosphorylation control

P

Role of fine control: “feedback inhibition” for homeostasis

Role of fine control: “feedback inhibition” for homeostasis

C. Role of free energy in metabolism

Chemical and physical reactions are characterized bya change in “free energy” (!G); this change in freeenergy includes any difference (between substratesand products) in concentration, potential energy(chemical bonds) and probability (order, entropy).

Higherconcentration

Less probablearrangement

Less stablebonds

ΔG < 0

ΔG > 0

Free energy measures the tendency of a reaction to proceed. ! If !G < 0, the reaction runs spontaneously. ! If !G > 0, the reaction will not run spontaneously

(runs backward). ! If !G = 0, the reaction is at equilibrium. For many biological reactions, !G > 0 Movement: physical work (contraction—potential energy; flagellation, protoplasmic streaming--improbable, orderly) Transport: chemical work (active transport against a concentration gradient--improbable, orderly) Synthesis of fatty acids, amino acids, etc. (unstable bonds; improbable, orderly)

Coupled reactions

Because !G > 0, these reactions will not proceed bythemselves.

They can proceed spontaneously only if they arecoupled to (made to work with) a reaction with!G < 0 so that the total !G < 0 for the two reactions(for coupled reactions, you can add individual !Gs tofind total !G).

To say that “life needs energy” means that life needsa source of substrates for reactions that have!G < 0, that “release free energy”.

NADH + H+ + 1/2 O2 −−> NAD+ + H2O

ΔG = -52 Kcal/mol

ATP and NADH are often considered “sources ofenergy” for the cell.

Mechanisms for generating ATP and NADH are ofgeneral importance.

! Respiration! Photosynthesis