8.1 - Metabolism

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

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

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

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

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.