Cellular Processes are not Spontaneous
- Within cells, cellular processes are not necessarily thermodynamically favorable (that is, they are endergonic). This might be because of more favorable structure.
- There are two ways to carry out the processes within the cell even if they are endergonic
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By having more reactants. In effect, this increases the equilibrium constant so that the forward reaction is favored.
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By introducing additional energy. This can be done by using common intermediates in metabolic processes and coupling endergonic reactions with exergonic reactions .
Usually, the common intermediate comes from ATP since the reaction of ATP Hydrolysis is more or less spontaneous.
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- Metabolism relies on the cell not being in equilibrium. Irreversible reactions drive metabolism forward, and the cell itself can obtain more reactants to maintain a steady state.
How do Enzymes Catalyze?
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On their own, cellular processes will take hundreds of thousands of years to complete. This requires lowering activation energy.
- A change in temperature. However, this has the effect of denaturing proteins and damaging cell structure.
- Enzymes
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Enzymes cause their substrates to be very reactive without requiring very high energy inputs. This is done by binding to reactants more tightly when they are ready to be converted to products.
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The enzyme then performs any of the following:
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Reorienting the substrates correctly facilitate the reaction
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By altering the electrostatic configuration through the side chains in the enzyme through:
- Having an environment within the enzyme that has a suitable pH (usually through the enzyme’s cofactors).
- By producing a temporary covalent bond between the enzyme and the substrate
- By facilitating hydrolysis
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By exerting a physical stress through an induced fit where the enzyme forces a change to allow the substrate to fit in the active sites.
In the process, the shape of the enzyme is changed to relieve the strain.
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Once the reaction proceeds, the enzyme has lower affinity for the products which are then expelled.
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Enzymes have high specificity— they only react to certain substrates.
What Influences Enzyme Activity?
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Under the Michaelis-Metten Model we can gather the following insights:
- At low substrate concentrations, the amount of substrate limits the reaction rate since there are few substrates to carry out the reaction.
- At high substrate concentrations, the amount of enzyme limits the reaction rate since all enzyme molecules are now being utilized with some substrates not being used.
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The presence of inhibitors that decrease the activity of the enzyme.
- Irreversible inhibitors bond very tightly to an enzyme and forms a covalent bond with one of its residues.
- Reversible inhibitors bind to the enzyme loosely and are easily displaced
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Competitive inhibitors compete for the substrate for access to the active site.
They resemble the substrate and can bind to the enzyme, preventing the reaction by removing the reactants.
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Noncompetitive inhibitors act as false active sites and bind to substrates.
They resemble the active site and prevent the reaction by removing the catalyst.
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Regulating enzyme activity has its own applications
- Some toxins work under the principle of irreversibly inhibiting enzymes in the cell.
- Drugs work by reversibly inhibiting certain enzymes to prevent certain reactions.
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Enzyme activity can be altered in two ways
- Covalent Modification - the catalytic activity of the enzyme can be changed by modifying its conformation through transfering phosphate groups.
- Allosteric Modulation - the catalytic activity of the enzyme cna be changed through compounds that bind to allosteric sites. This induces a ripple effect that alters the conformation of the active site.
- Allosteric modulation can induce feedback inhibition by inhibiting the reaction when there is a surplus of byproducts.
Links
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Metabolic Processes within the Cell - for more information on cellular processes.
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Karp Ch. 3 - select chapters for enzymes:
- 3.2 - 3.3 - for the thermodynamics of the cell
- 3.5 - 3.6 - for understanding enzyme activity.
- 3.7 - for understanding inhibitors and enzyme kinetics.
- 3.12 - to see how enzymes can be altered.