Enzymes: Basic concepts and kinetics Flashcards
What are most catalysts in biological systems
Enzymes
Enzymes
Nearly all are proteins. Highly specific. Can enhance reaction rates by 10^6 or more. They do not alter reaction equilibria, they increase the rate at which equilibrium is obtained.
Cofactors
Many enzymes require these for activity. Can be metal ions or coenzymes (small, vitamin-derived organic molecules).
When can a reaction take place spontaneously
If change in free energy (^G) is negative
The standard free energy change (^G*)
The free -energy change of a reaction that takes place when reactants and products are at unit activity
The standard free-energy change at pH 7
^G*’
How do enzymes serve as catalysts?
By decreasing the free energy of activation of chemical reactions
How do enzymes accelerate reactions?
By providing a reaction pathway in which the transition state (the highest energy species) has a lower free energy and so is more rapidly formed than in the uncatalyzed reaction
What is the 1st step in catalysis?
The formation of an enzyme-substrate complex
How are substrates bound to enzymes
At active site clefts from which water is largely excluded when the substrate is bound
Where does the specificity of enzyme-substrate interactions arise from?
Mainly from hydrogen bonding, which is directional, and from the shape of the active site, which rejects molecules which do not have a complementary shape.
How do enzymes facilitate formation of the transition state?
By a dynamic process in which the substrate binds to specific conformations of the enzyme, along with conformational changes at active sites that result in catalysis
The Michaelis-Menten model
Describes the kinetic properties of many enzymes. An enzyme (E) combines with substrate (S) to form enzyme-substrate (ES) complex, which can form a product (P) or dissociate into E and S.
The rate of the formation of product Vo is given by the Michaelis-Menten equation….
Vo = Vmax([S]/([S] + Km)).
Vmax = the reaction rate when the enzyme is fully saturated with substrate. Km = The Michaelis constant. The substrate concentration at which the reaction rate is half maximal
kcat/Km
Ratio providing a measure of enzyme efficiency and specificity
3 facts about allosteric enzymes
- Catalytic activity can be regulated
- Do not conform to Michaelis-Menten kinetics
- Have multiple active sites
Allosteric enzyme active sites
Display cooperativity, as evidenced by a sigmoidal (s-shaped curve) dependence of reaction velocity on substrate concentration.
Enzyme inhibition
Specific small molecules or ions can inhibit even nonallosteric enzymes
Irreversible inhibition
Inhibitor is covalently linked to enzyme or bound so tightly that its dissociation from the enzyme is v.slow.
How can enzymes active site be mapped?
With covalent inhibitors
Reversible inhibition
More rapid and less stable interaction between enzyme and inhibitor.
Competitive inhibitor
Prevents the substrate from binding the active site. Reduces the reaction velocity by diminishing the proportion of enzyme molecules that are bound to substrate
How can competitive inhibition be overcome?
Raise the substrate concentration. Cannot overcome uncompetitive and noncompetitive in this way.
Uncompetitive inhibition
Inhibitor combines only with ES complex.
Noncompetitive inhibition
Inhibitor decreases turnover number.
Transition state
Enzyme binds transition state more tightly than substrate
Transition-state analogs
Stable compounds that mimic key features of this highest energy species. Potent and specific inhibitors of enzymes. Used as antigens, or immunogens, in generating catalytic antibodies.
Enzymes can be studied at level of a single molecule (in singulo)
Yield info that is difficult to gain in studies of populations of molecules. Reveal a distribution of enzyme characteristics rather than an average value.
