Enzymes as Biological Catalysts

   Enzymes as Biological Catalysts

•Enzymes are proteins that increase the rate of reaction by lowering the energy of activation
•They catalyze nearly all the chemical reactions taking place in the cells of the body
•Enzymes have unique three-dimensional shapes that fit the shapes of reactants (substrates)

Naming Enzymes

•The name of an enzyme identifies the reacting substance
– usually ends in –ase
•For example, sucrase catalyzes the hydrolysis of sucrose
•The name also describes the function of the enzyme
•For example, oxidases catalyze oxidation reactions
•Sometimes common names are used, particularly for the digestion enzymes such as pepsin and trypsin
•Some names describe both the substrate and the function
•For example, alcohol dehydrogenase oxides ethanol

Classification of Enzymes

•Enzymes are classified according to the type of reaction they catalyze:
Class Reactions catalyzed
Oxidoreductases Oxidation-reduction
Transferases Transfer groups of atoms
Hydrolases Hydrolysis
Lyases Add atoms/remove atoms to/from a double bond
Isomerases Rearrange atoms
Ligases Use ATP to combine molecules
•Oxidoreductases, Transferases and Hydrolases
•Lyases, Isomerases and Ligases

Active Site of an Enzyme

•The active site is a region within an enzyme that fits the shape of substrate molecules
•Amino acid side-chains align to bind the substrate through H-bonding, salt-bridges, hydrophobic interactions, etc.
•Products are released when the reaction is complete (they no longer fit well in the active site)

Enzyme Specificity

•Enzymes have varying degrees of specificity for substrates
•Enzymes may recognize and catalyze:
– a single substrate
– a group of similar substrates
– a particular type of bond

Lock-and-Key Model

•In the lock-and-key model of enzyme action:
– the active site has a rigid shape
– only substrates with the matching shape can fit
– the substrate is a key that fits the lock of the active site
•This is an older model, however, and does not work for all enzymes

Induced Fit Model

•In the induced-fit model of enzyme action:
– the active site is flexible, not rigid
– the shapes of the enzyme, active site, and substrate adjust to maximumize the fit, which improves catalysis
– there is a greater range of substrate specificity
•This model is more consistent with a wider range of enzymes

Enzyme Catalyzed Reactions

•When a substrate (S) fits properly in an active site, an enzyme-substrate (ES) complex is formed:
E + S D ES
•Within the active site of the ES complex, the reaction occurs to convert substrate to product (P):
ES ® E + P
•The products are then released, allowing another substrate molecule to bind the enzyme
– this cycle can be repeated millions (or even more) times per minute
•The overall reaction for the conversion of substrate to product can be written as follows:
E + S D ES ® E + P
•Example of an Enzyme Catalyzed Reaction
•The reaction for the sucrase catalyzed hydrolysis of sucrose to glucose and fructose can be written as follows:
E + S D ES ® E + P1 + P2
where E = sucrase, S = sucrose, P1 = glucose and P2 = fructose

Isoenzymes

•Isoenzymes are different forms of an enzyme that catalyze the same reaction in different tissues in the body
– they have slight variations in the amino acid sequences of the subunits of their quaternary structure
•For example, lactate dehydrogenase (LDH), which converts lactate to pyruvate, consists of five isoenzymes

Diagnostic Enzymes

•The levels of diagnostic enzymes in the blood can be used to determine the amount of damage in specific tissues
•Temperature and Enzyme Activity
•Enzymes are most active at an optimum temperature (usually 37°C in humans)
•They show little activity at low temperatures
•Activity is lost at high temperatures as denaturation occurs
•pH and Enzyme Activity
•Enzymes are most active at optimum pH
•Amino acids with acidic or basic side-chains have the proper charges when the pH is optimum
•Activity is lost at low or high pH as tertiary structure is disrupted
•Optimum pH for Selected Enzymes
•Most enzymes of the body have an optimum pH of about 7.4
•However, in certain organs, enzymes operate at lower and higher optimum pH values
•Enzyme Concentration and Reaction Rate
•The rate of reaction increases as enzyme concentration increases (at constant substrate concentration)
•At higher enzyme concentrations, more enzymes are available to catalyze the reaction (more reactions at once)
•There is a linear relationship between reaction rate and enzyme concentration (at constant substrate concentration)
•Substrate Concentration and Reaction Rate
•The rate of reaction increases as substrate concentration increases (at constant enzyme concentration)
•Maximum activity occurs when the enzyme is saturated (when all enzymes are binding substrate)
•The relationship between reaction rate and substrate concentration is exponential, and asymptotes (levels off) when the enzyme is saturated

Enzyme Inhibitors

•Inhibitors (I) are molecules that cause a loss of enzyme activity
•They prevent substrates from fitting into the active site of the enzyme:
E + S D ES ® E + P
E + I D EI ® no P formed
•Reversible Inhibitors (Competitive Inhibition)
•A reversible inhibitor goes on and off, allowing the enzyme to regain activity when the inhibitor leaves
•A competitive inhibitor is reversible and has a structure like the substrate
– it competes with the substrate for the active site
– its effect is reversed by increasing substrate concentration
•Example of a Competitive Inhibitor
•Malonate is a competitive inhibitor of succinate dehydrogenase
– it has a structure that is similar to succinate
– inhibition can be reversed by adding succinate
•Reversible Inhibitors (Noncompetitive Inhibition)
•A noncompetitive inhibitor has a structure that is different than that of the substrate
– it binds to an allosteric site rather than to the active site
– it distorts the shape of the enzyme, which alters the shape of the active site and prevents the binding of the substrate
•The effect can not be reversed by adding more substrate
•Irreversible Inhibitors
•An irreversible inhibitor destroys enzyme activity, usually by bonding with side-chain groups in the active site

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