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MCAT Biochemistry: Enzymes

Terms in this set (65)

Enzymes
~ Biological catalysts
~ Remains enzyme
~ Increase rate by factor of 100, 1000 or even 10^12
Catalysts
~ Help reaction proceed at a much faster rate
~ Do not impact the thermodynamics of biological reaction
~ Equilibrium position does not change
Key Features of Enzymes
~ Lower the activation energy
~ Increase rate of reaction
~ Do not alter equilibrium constant
~ Are changes or consumed in reaction
~ pH and temp sensitive
~ Do not affect the overall delta G
~ Specific to reaction
Molecule the enzyme acts on...
~ Substrate
Enzyme Specificity
~ A given enzyme will only catalyze a single reaction or class of reactions with certain substrates
Enzymes classified into...
~ 6 categories
~ LIL HOT
Enzymes typically end in...
~ -ase
Oxidoreductases
~ Catalyze oxidation-reduction reactions
~ Transfer e- between biological molecules
~ Often have a cofactor (NAD+ and NADP+)
Reductant
~ Electron donor
Oxidant
~ Electron acceptor
Enzymes with dehydrogenase or reductase
~ Oxidoreductases
Enzymes with oxygen as final e- receptor
~ Include oxidase in their name
Transferases
~ Catalyze movement of a functional group from one molecule to another
~ Kinases are in this class
Kinases
~ Catalyze transfer of phosphate group (generally ATP)
Hydrolases
~ Catalyze the breaking of compound into two molecules using addition of water
~ Normally named only for their substrate
Lyases
~ Catalyze the cleavage of single molecule into two products
~ Do not require water and do not act like oxidoreductases
~ Can also do the reverse --> synthases
Isomerases
~ Catalyze the rearrangement of bonds within a molecule
~ Can be classified as O, T or L depending on mechanism
~ Catalyze reactions between stereoisomers and constitutional isomers
Ligases
~ Catalyze addition reactions between lg. similar molecules
~ Synth with smaller molecules --> lyases
Impact on Activation Energy
~ Endergonic
~ Exergonic
Endergonic
~ One that requires energy input
Exergonic
~ One in which energy is given off
Lower Activation Energy
~ Make it easier to reach transition state
Enzyme-Substrate Binding
~ Physical interaction between the enzyme and substrate
Active Site
~ Location within enzyme where substrate is held
~ Assumes a defined spatial arrangement which dictates enzyme specificity
~ Hydrogen bonding, ionic interactions and transient covalent bonds stabalize
Lock and Key Theory
~ Active site is already in appropriate confirmation of substrate to bind
~ No alteration
Induced Fit Model
~ Adjust to fit each other well
~ Substrate induced a change in shape of enzyme
~ Requires energy (endergonic)
~ Releasing substrate (exergonic)
~ Wrong substrate won't induce conformational change
Cofactors or Coenzymes
~ Nonprotein
~ Small
~ Bind to AS and participate
~ Carry charge throughh ionization, prot, deprot
~ Low [ ] in cell
~ Tightly bound are necessary for enzyme function
Apoenzymes
~ No cofactors
Holoenzymes
~ Contain cofactors
Cofactors
~ Inorganic or metal ions
~ Ingested dietary minerals
Coenzymes
~ Small organic groups
~ Vitamins or derivatives of vitamins
B1
Thiamine
B2
Riboflavin
B3
Niacin
B5
Pantothenic Acid
B6
Pyridoxal phosphate
B7
~ Biotin
B9
~ Folic Acid
B12
~ Cyanocobalamin
Saturation
~ All active sites are occupied
Maximum Velocity
~ At saturation point
~ Cannot go any faster
Michaelis-Menten Plot of Enzyme Kinetics
Rate of Reaction
~ ES form at rate k1
~ ES dissociate at rate k2
~ Turn into E+P at rate k3
Michaelis-Menten Equation
~ Velocity related to concentration
Km
~ [Substrate] at which 1/2 of enzyme's AS are full
~ Michaelis constant
~ Sometimes measured affinity of enzyme for its substrate
~ Cannot be altered
Higher Km
~ Has lower affinity for its substrate because it requires a higher substrate [ ] to be 1/2 saturated
Lineweaver-Burk Plots
~ Double reciprocal of MM
~ Help determine the types of inhibition that an enzyme is experiencing
Cooperativity
~ Enzymes that have multiple subunits and active sites
Enzymes exist in 2 states
~ Low-affinity tense state
~ High-affinity relaxed state
~ Binding transfers from L to H
Enzymes and Temperature
~ Tend to double rate with every 10 degree increase
~ After optimal temp is reached activity sharply falls off
pH
~ Affects ionization of AS
Acidemia
~ pH less than 7.35
Salinity
~ Disrupt hydrogen and ionic binds
~ Cause a partial change in conformation
Feedback Regulation
~ Enzymes regulated by products further down the metabolic pathway
Feedforward Regulation
~ Enzymes regulated by intermediates that precede them
Feedback Inhibition or Negative Feedback
~ Helps maintain homeostasis
~ Once we have enough we turn the pathway off
~ Competitively inhibit
Reversible Inhibition
~ Competitive
~ Noncompetitive
~ Mixed
~ Uncompetitive
Competitive Inhibition
~ Occupancy of the AS
~ Substrates cannot access enzymatic binding sites
~ Overcome by adding more substrate
Noncompetitive Inhibition
~ Bind to allosteric site (non-catalytic) change enzyme conformation
~ Decreases measured value of vmax
Mixed Inhibition
~ Inhibitor can bind to either the enzyme (increases km) or the ES complex (decreases km)
~ Different affinity
~ Do not bind to AS
Uncompetitive Inhibition
~ Bind only to ES
~ Lock substrate in enzyme
~ Prevent release
~ At an allosteric site
~ Conformational change
Irreversible Inhibition
~ AS made unavailable
~ Enzyme is permanently altered
~ Not easily overcome or reversed
~ ex. Aspirin
Allosteric Enzymes
~ Multiple binding sites
~ AS and allosteric sites
~ Alt. between active and inactive
~ Molecules that bind to allosteric site will either be activators (make it more available) or inhibitors (make it less available)
Covalently Modified Enzymes
~ Act. or deact. by phos or dephos
~ Cannot predict which will do which
Zymogens
~ To avoid danger, these enzymes and many others are secreted as inactive
~ Apoptotic enzymes similar regulation
~ Suffix: -ogen
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