Chapter 3 Flashcards
Enzymes as Proteins
-Enzymes are biological catalysts ‘Biological’ because they function in living systems ‘Catalysts’ because they speed up the rate of chemical reactions without being used up or changed -Enzymes are also globular proteins -Critical to the enzyme’s function is the active site where the substrate binds -Metabolic pathways are controlled by enzymes in a biochemical cascade of reactions -Virtually every metabolic reaction within living organisms is catalysed by an enzyme – enzymes are therefore essential for life to exist
types of enzymes
Mode of Enzyme Action
- Enzymes have an active site where specific substrates bind forming an enzyme-substrate complex
- The active site of an enzyme has a specific shape to fit a specific substrate
- Substrates collide with the enzymes active site and this must happen at the correct orientation and speed in order for a reaction to occur
features that can denatures
-Extremes of heat or pH can change the shape of the active site, preventing substrate binding – this is called denaturation
The specificity of an enzyme
is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)
-The shape of the active site (and therefore the specificity of the enzyme) is determined by the complex tertiary structure of the protein that makes up the enzyme:
–Proteins are formed from chains of amino acids held together by peptide bonds –The order of amino acids determines the shape of an enzyme
–If the order is altered, the resulting three-dimensional shape chang
The temporary formation of an enzyme-substrate complex
-An enzyme-substrate complex forms when an enzyme and its substrate join together -The enzyme-substrate complex is only formed temporarily, before the enzyme catalyses the reaction and the product(s) are released
Enzyme reactions can either be
catabolic or anabolic
catabolic
Catabolic reactions involve the breakdown of complex molecules into simpler products, which happens when a single substrate is drawn into the active site and broken apart into two or more distinct molecules Examples of catabolic reactions include cellular respiration and hydrolysis reactions
Anabolic
Anabolic reactions involve the building of more complex molecules from simpler ones by drawing two or more substrates into the active site, forming bonds between them and releasing a single product Examples of anabolic reactions include protein synthesis and photosynthesis
Enzymes work by lowering the activation energy of a reaction
-Enzymes speed up chemical reactions because they influence the stability of bonds in the reactants -The destabilisation of bonds in the substrate makes it more reactive -Enzymes work by lowering the activation energy of a reaction and in doing so they provide an alternative energy pathway
activation energy
-All chemical reactions are associated with energy changes -For a reaction to proceed there must be enough activation energy -Activation energy is the amount of energy needed by the substrate to become just unstable enough for a reaction to occur and for products to be formed
The lock-and-key hypothesis
-Enzymes are globular proteins -This means their shape (as well as the shape of the active site of an enzyme) is determined by the complex tertiary structure of the protein that makes up the enzyme and is therefore highly specific -In the 1890’s the first model of enzyme activity was described by Emil Fischer: -He suggested that both enzymes and substrates were rigid structures that locked into each other very precisely, much like a key going into a lock -This is known as the ‘lock-and-key hypothesis’ -This was later modified and adapted to our current understanding of enzyme activity, permitted by advances in techniques in the molecular sciences
The induced-fit hypothesis
-The modified model of enzyme activity is known as the ‘induced-fit hypothesis’ -Although it is very similar to the lock and key hypothesis, in this model the enzyme and substrate interact with each other: -The enzyme and its active site (and sometimes the substrate) can change shape slightly as the substrate molecule enters the enzyme -These changes in shape are known as conformational changes -This ensures an ideal binding arrangement between the enzyme and substrate is achieved -This maximises the ability of the enzyme to catalyse the reaction
