Chapter 6 Flashcards
True or false: Enzymes are powerful biological catalysts, responsible for the harmonious interplay of ALL cellular processes
True!
Why do enzymes have a high degree of specificity?
Their specialized pockets called active sites (aka binding sites)
The shape, size, and chemical properties of the active site are complementary to the substrate, meaning that only specific molecules (substrates) can fit into this site.
Describe the two concepts that explain the catalytic power of enzymes
- They bind most tightly to the transition state of the catalyzed reaction and use binding energy to lower the activation barrier.
- Evolution has organized or finely-tuned the design of active sites, such that multiple mechanisms of chemical catalysis can occur simultaneously.
Explain how an enzyme uses binding energy to lower the activation barrier
Ultimately derived from the free energy released in forming many weak bonds between an enzyme and its substrate.
This binding energy contributes to specificity as well as to catalysis.
Weak interactions are optimized in the reaction transition state; enzyme active sites are complementary not to the substrates per se but to the transition states through which substrates pass as they are converted to products
Describe Enzyme regulation: (reversible) covalent modifications
Phosphorylation and protein kinases/phosphatases.
Phosphorylation is the addition of a phosphate group (PO₄²⁻) to an enzyme, typically at serine, threonine, or tyrosine residues. It is catalyzed by kinases (enzymes that add phosphate groups).
Dephosphorylation is the removal of a phosphate group, typically catalyzed by phosphatases
Describe Enzyme regulation: allosteric modulators
Enzyme regulation by allosteric modulators involves the binding of molecules (called allosteric effectors or modulators) to a site on the enzyme that is distinct from the active site, known as the allosteric site.
The binding of these molecules induces conformational changes in the enzyme that can either enhance (activate) or inhibit (deactivate) its activity
Describe Enzyme regulation: proteolytic (de)activation
An enzyme is initially synthesized as an inactive precursor, known as a zymogen or proenzyme, and is activated by the irreversible removal of a portion of its polypeptide chain.
This process involves proteolysis, which refers to the cleavage of peptide bonds, typically at specific sites in the enzyme precursor.
This regulation allows the enzyme to be activated only when needed and in the appropriate cellular location, ensuring that its activity is tightly controlled
The activation of a zymogen is typically irreversible, meaning that once the cleavage has occurred, the enzyme remains active
Describe Enzyme regulation: noncovalent interactions with other (regulatory) proteins
These interactions typically involve hydrogen bonds, ionic interactions, van der Waals forces, and hydrophobic interactions.
Regulatory proteins can act as activators or inhibitors of the target enzyme. They often bind to a site on the enzyme distinct from the active site, resulting in a conformational change in the enzyme that alters its activity.
could be allosteric, feedback regulation, etc.
Describe Enzyme regulation: pH
Enzymes have an optimal pH at which they function most efficiently, and deviations from this optimal pH can lead to a decrease in enzymatic activity or complete inactivation
The optimal pH is determined by the enzyme’s structure and the environment in which it needs to perform its function
Acidic environments (low pH) can protonate amino acid residues, while basic environments (high pH) can deprotonate them.
These changes can disrupt the enzyme’s active site, reducing its ability to bind substrates or catalyze reactions effectively.
What is a cofactor?
Cofactors assist in the formation of the enzyme-substrate complex participate directly in the chemical reaction.
Inorganic metal ions such as Zn²⁺, Fe²⁺, Mg²⁺, or Cu²⁺
Assist in stabilizing the enzyme’s structure or participate in the catalytic process by acting as electron carriers, cofactors in redox reactions, or coordinating with substrates
What is a coenzyme?
Organic molecules that bind to the enzyme, often in a non-covalent manner, and are required for enzyme activity.
Cofactors often act as carriers of chemical groups or electrons
Define prosthetic group
A coenzyme or metal ion that is very tightly or even covalently bound to the enzyme protein is called a prosthetic group.
Draw a free energy diagram of a catalyzed and uncatalyzed reaction. Be sure to include ground and transition state; activation energy; reaction intermediates; rate-limiting step; binding energy
Check Textbook for correct answer
define DeltaG10
A biochemical standard free-energy change.
AKA the overall standard free-energy change in the direction S–>P
What is DeltaG(with little tree)
Activation energy
Define transition state
At the top of the energy hill is a point at which decay to the S or P state is equally probable (it is downhill either way).
The transition state is not a chemical species with any significant stability and should not be confused with a reaction intermediate (such as ES or EP). It is simply a fleeting molecular moment in which events such as bond breakage, bond formation, and charge development have proceeded to the precise point at which decay to either substrate or product is equally likely
What does a higher activation energy correspond to?
a slower reaction
Define reaction intermediates
A reaction intermediate is
any species on the reaction pathway that has a finite chemical lifetime.
the ES and EP complexes occupy valleys in the catalyzed reaction coordinate diagram
Define rate limiting step
the overall rate is determined
by the step (or steps) with the highest activation energy; this is called the rate-limiting step. In a simple case,
the rate-limiting step is the highest-energy point in the diagram for interconversion of S and P
What is binding energy?
The energy derived from enzyme-substrate interaction is called binding energy, deltaGB. Its significance extends beyond a simple stabilization of the enzyme-substrate interaction. Binding energy is a major source of free energy used by enzymes to lower the activation energies of reactions
Define Keq
The equilibrium constant.
Keq= [P]/[S]
The the relationship between Keq
and deltaG10 can be described by the expression:
deltaG10= -RT ln Keq
Describe how rate constant and rate equation are related for a first order reaction
The rate of any reaction is determined by the concentration of the reactant (or reactants) and by a rate constant, usually denoted by k.
For the unimolecular reaction S–>P, the rate (or velocity) of the reaction,
V—representing the amount of S that reacts per unit time—is expressed by a rate equation:
V = k[S]
In this reaction, the rate depends only on the concentration of S
What is a first order reaction?
The rate depends only on the concentration of S