OXIDATION & REDUCTION INVOLVES ELECTRON TRANSFER

Organic material

Burn

Single Step

Heat

Direct addition of oxygen

Metabolism

Multiple Steps

Enzymes

No direct addition of

oxygen

Cellular process Vs Burning

Oxidation - reduction

• Oxidation is loss of electrons

• Reduction is gain of electrons

– Oxidation is always accompanied by reduction

• The total number of electrons is kept constant

• Oxidizing agents oxidize and are themselves

reduced

• Reducing agents reduce and are themselves

oxidized

Follow the electrons

Combustion of sugar (or respiration).

• The number of electrons is conserved

• Oxidation and reduction always occurs

simultaneously

• Glucose reacts with molecular oxygen to produce

carbon dioxide and water.

• The carbon atoms in glucose are oxidized. That is,

they lose electron and go to a higher oxidation state.

• The oxygen atoms in molecular oxygen are reduced.

That is, they add electrons and go to a lower

oxidation state

Reaction

Formation of Polar Covalent Bond

Hydrogenation & Dehydrogenation

Increase C-H Bonds

(Hydrogenation/Reduction)

Decrease C-H Bonds (Dehydrogenation/Oxidation)

HOW ENZYMES FIND THEIR SUBSTRATE: THE ENORMOUS RAPIDITY OF

MOLECULAR MOTIONS

How do E & S find each other?

• Enzyme Substrate reaction rate = (1000 substrate

molecules/ Seconds )

• Thus Enzyme bind to substrate for a milliseconds

• High Molecular level motions due to heat energy

– Translational motion

– Vibration

– Rotations

Meaured by spectroscopic techniques

Diffusion & Random walk

• Molecules are also in constant translational motion,

which causes them to explore the space inside the cell

very efficiently by wandering through it-a process

called diffusion.

• Every molecule in a cell collides with a huge number

of other molecules each second. As the molecules in a

liquid collide and bounce off one another, an

individual molecule moves first one way and then

another, its path constituting a random walk

• Average net distance that each molecule travels (as

the crow flies) from its starting point is proportional

to the square root of the time

– 1second to travel 1 µm

– 4 seconds to travel 2 µm

– 100 seconds to travel 10 µm

• A small organic molecule, for example, takes only

about one-fifth of a second on average to diffuse a

distance of 10 pm

Random Walk

Rate of Encounter

• The rate of encounter of each enzyme molecule with

its substrate will depend on the concentration of the

substrate molecule. For example,

– some abundant substrates are present at a concentration of

0.5 mM. Since pure water is 55.5 M, there is only about

one such substrate molecule in the cell for every 10s water

molecules.

• Active site on an enzyme molecule that binds this

substrate will be bombarded by about 500,000

random collisions with the substrate molecule per

second.

Stronger the binding of the enzyme &

substrate, the slower their rate of

dissociation.• Once an enzyme and substrate have collided and

smuggled together properly at the active site, they

form multiple weak bonds with each other that persist

until random thermal motion causes the molecules to

dissociate again

• when two colliding molecules have poorly matching

surfaces, they form few non-covalent bonds and their

total energy is negligible compared with that of

thermal motion. Two molecules dissociate as rapidly

as they come together, preventing incorrect Link

between an enzyme and the wrong substrate