8.1 - Metabolism

Page 1

Metabolic pathways

a sequence of small steps that form chemical reactions

most metabolic pathways involve a chain of reactions

example: converting phenylalanine into fumarate and acetoacetate

some metabolic pathways form a cycle rather than a chain

the end product of one reaction is the reactant that starts the pathway

example: Calvin cycle and Krebs cycle

Page 2

Enzymes and activation energy

substrates pass through a transition to be converted into products

energy is required to reach the transition stage

this is called the activation energy

the activation energy is used to break or weaken bonds in the substrates

when an enzyme catalyzes a reaction, the substrate binds to the active site and is altered to reach the transition state, it is then converted to products, which separate from the active site

this binding lowers the overall energy level of the transition state

the activation energy is lowered

the net amount of energy released by the reaction is unchanged

but the rate of reaction increases, typically by a factor of a million or more

Page 3

Types of enzyme inhibitors

chemical substances bind to enzymes and reduce their activity

they are known as inhibitors

the two main types:

competitive inhibitors

interfere with the active site

the substrate cannot bind

non-competitive inhibitors

bind at another location

change the shape of the enzyme

Page 4

Effects of enzyme inhibitors

competitive inhibitor

the concentration of substrate begins to exceed the amount of inhibitor, the maximum rate of the uninhibited enzyme can be achieved but it takes a much higher concentration of substrate to achieve this maximum rate

non-competitive inhibitor

the enzyme doesn't reach the same maximum rate because the binding of the inhibitor prevents some of the enzymes from being able to react regardless of substrate concentration

same pattern as the normal enzyme

it takes approximately the same concentration of enzyme to reach the maximum rate, but the maximum rate is lower than the uninhibited enzyme

Page 5

End-product inhibition

allosteric interactions: when enzymes are regulated by chemical substances that bind to special sites on the enzymes away from the active site

allosteric site: where these interactions take place

in many cases, the enzyme that is regulated catalyzes one of the first reactions in a metabolic pathway and the substance that binds to the allosteric site is the end product of the pathway

the end product acts as an inhibitor

the pathway works rapidly in cells with a shortage of end product but can be switched off completely in cells where there is an excess

reactions don't often go to completion, an equilibrium position is reached with a characteristic ratio of substrates and products

so if the concentration of products increases, a reaction will eventually slow down and stop, this effect reverberates back through a metabolic pathway when the end product accumulates

end-product inhibition prevents this build-up of intermediate products

example: through a series of five reactions, threonine is converted into isoleucine; as the concentration of isoleucine builds up, it binds to the allosteric site of the first enzyme in the chain, threonine deaminase, acting as a non-competitive inhibitor.