Biochemistry Ch2 Flashcards
What are the key features of enzymes
- Lower the activation energy
- Increase the rate of the reaction
- do not alter the equilibrium constant
- Are not changed or consumed in the reaction
- Are pH and temperature sensitive
- so not affect the overall change in energy
- are specific for a particular reaction or class of reactions
Oxidoreductase
- Catalyze oxidation-reduction reactions
- Transfer of electrons between biological molecules
- Often involve a cofactor like electron carriers
- e donor is the reductant and e acceptor is the oxidant
Transferases
Catalyze the movement of a functional group from one molecule to another
Kinases
Catalyze the transfer of a phosphate group, generally form ATP, to another molecule
- type of transferase
Hydrolases
Catalyze the breaking of a compound into two molecules using the addition of water
- naming only after the substrate
Lyases
- Catalyze the cleavage of a single molecule into two products
- Does not require water and doesn’t act as an oxidoreductase
- Most reversible
- opposite is a synthase
Isomerases
Catalyze the rearrangement of bonds within a molecule
Ligases
Catalyze addition or synthesis reactions
- generally between larger similar molecules
- often require ATP
Cofactors and coenzymes
Bind to the active site of the enzyme and participate in the catalysis of the reaction
- Cofactors generally inorganic molecules or metal ions
- Coenzymes are small organic groups (majority vitamins)
Apoenzymes
Enzymes without their cofactors
Holoenzymes
Enzymes with their cofactors
Prosthetic groups
Tightly bound cofactors or coenzymes that are necessary for enzyme function
What are the names of the B vitamins
Thiamine
riboflavin
niacin
pantothenic acid
pyridoxal phosphate
biotin
folic acid
cyanocobalamin
Lineweaver-Burk Plots
A double reciprocal graph of the Michaelis-Menten equation
- The x-intercept gives the value of -1/Km
- The y-intercept fives the value of 1/vmax
Cooperativity
Cooperative enzymes have multiple subunits and multiple active sites
- Subunits and enzymes may exist low-affinity tense state (T) or high-affinity relaxed state (R)
- Binding of the substrate encourages the transition of other subunits from the T state to the R state, which increases the likelihood of substrate binding by these other subunits and vice versa
Hill’s Coefficient
Indicates the nature of binding by the molecule
- >1, positively cooperative binding is occurring, such that after one ligand is bound the affinity of the enzyme for further ligands increase
- <1 negatively cooperative binding is occurring, such that after one ligand is bound the affinity of the enzyme for further ligands decreases
- =1, the enzyme does not exhibit cooperative behaviour
Effect of temperature on enzymes
- Enzyme-catalyzed reactions double in velocity every 10 degrees increase in temp until the optimum temperature is reached
- after this activity falls off sharply, as the enzyme will denature at higher temperatures
What is the optimal temperature of the human body
37 degrees C
pH effect of enzymes
pH affects the ionization of the active site and can lead to denaturation of the ezyme
What is the optimal pH of the human body
7.4
Salinity effect on enzymes
- Not generally physiologically significant
- Can change enzyme activity in vitro
- Disrupts hydrogen and ionic bonds, causing partial change in the conformation of the enzyme and in some cases causing denaturation
Competitive Inhibition
Occupancy of the active site
- substrates cannot access enzymatic binding sites
- Can be overcome by adding more substrate so that the substrate-to-inhibitor ratio is higher
- Increases the measured value of Km because the substrate concentration has to be higher to reach half the max velocity in the presence of the inhibitor
Noncompetitive Inhibition
Bind to an allosteric site instead of the active site, which induces a change in enzyme conformation
- Cannot be overcome by adding more substrate
- Adding a noncompetitive inhibitor decreases the measured value of vmax because less enzyme available to react
Mixed inhibition
When an inhibitor can bind to either the enzyme or the enzyme-substrate complex, but has different affinity for each
- If the inhibitor preferentially binds to the enzyme, it increases the Km value
- If the inhibitor preferentially binds to the enzyme-substrate complex, it lowers the Km value
- In either case Vmax is decreased
Uncompetitive Inhibition
Binds only to the enzyme-substrate complex and essentially lock the substrate in the enzyme, preventing its release
- Conformational change that allows the uncompetitive inhibitor to bind
- Lower Km (increased affinity) and increased vmax
Irreversible Inhibition
The active site is made unavailable for a prolonged period of time, or the enzyme is permanently altered
Allosteric Enzymes
Alternate between active and inactive forms
- Allosteric activators or allosteric inhibitors can bind to the allosteric sites
Covalently Modified Enzymes
Enzymes can be activated or deactivated by phosphorylation or dephosphorylation
- Can also undergo glycosylation, the covalent attachment of sugar moieties
Zymogens
Secreted in an inactive form and must be activated
- Contain a catalytic domain and regulatory domain
- The regulatory domain must be either removed or altered to expose the active site
