BMCB 658A EXAM 2

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91 Terms

1
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Which of the following is most directly related to the speed of a reaction?
The ΔG0‡ of the reaction
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Why can the breakdown of A T P power cellular processes?
As an exergonic reaction, it releases energy to spare.
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∆G < 0
Spontaneous exergonic
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∆G > 0
Nonspontaneous endergonic
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_____ is a measure of a system’s instability, its tendency to change to a more stable state.
Free energy
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During a spontaneous change free energy ….. and the stability of a system ….
decreases, increases
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∆G = 0
Equilibrium
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What conditions are necessary for the free energy change to predict the spontaneity of a reaction?
Free energy change can be used to predict spontaneity of reaction under the conditions of constant TEMPERATURE and PRESSURE
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Gibbs free energy equation
∆G = ∆H - T∆S
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If the ∆G is negative than the ….
reaction is spontaneous
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Standard states
Standard set of conditions used for comparisons of chemical reactions
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If \[H+\] = 1 M then
pH=0
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Standard state for \[H+\] is
\[H+\] = 10-7 M and pH = 7.0
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Modified standard state for free-energy change is
∆G°ʹ
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Spontaneous reaction occurs without
added energy
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How to make endergonic reactions proceed?
Couple endergonic reaction to an exergonic reaction
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Enzymes do not affect the change \n in free energy (ΔG); they only …..
speed up reactions that would eventually occur without them
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Standard free energy change (ΔG°)
Difference between energies of reactants and products \n under standard conditions
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Reaction rate depends on its …
activation energy (ΔG°‡),which is the energy input required to initiate a reaction
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Lock-and-key model
Substrate binds to that portion of the enzyme with a \n complementary shape
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Induced fit model
Binding of the substrate induces a change in the conformation of the enzyme that results in a complementary fit
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Proximity and orientation of the substrate …..
speed up the reaction
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Once enzyme and substrate bind (ES) and attain the transition state \n complex (EX‡) …..
catalysis can occur
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The active site can lower activation energy and speed up reactions by
* Orienting substrates correctly
* Straining substrate bonds
* Providing a favorable microenvironment
* Non-covalent bonding to the substrate
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The rate of catalysis can usually be sped up by
Increasing the substrate concentration
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Enzyme is saturated when …..
all enzyme molecules in a solution are bonded with substrate
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Competitive Inhibition
\n Decrease in enzymatic activity caused by binding of \n a substrate analogue to the active (catalytic) site \n

Inhibitor competes with the substrate for the active \n site on the enzyme
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Noncompetitive Inhibition
Form of enzyme inactivation in which a substance binds \n to a site other than the active site but distorts the \n active site to inhibit reaction

\
Involves two distinct binding sites, one for the substrate \n and one for the inhibitor
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Increasing substrate concentration cannot ….
overcome noncompetitive inhibition
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Uncompetitive Inhibition
Inhibitor can bind to the ES complex but not to the free E
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Allosteric enzyme
Oligomer whose biological activity is affected by other substances binding to it
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Allosteric effector
Substance that modifies the behavior of an allosteric enzyme by binding to it
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Binding of O2 to hemoglobin exhibits
positive cooperativity
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Instead of relying on heat, organisms carry out _____ to selectively speed up reactions
catalysis
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Enzymes are biological catalysts that are usually
globular proteins
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Enzyme and substrate must bind to form the
ES complex

\
E + S -→ ES
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First order
Description of a reaction whose rate depends on the first power of the concentration of a single reactant
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Second order
Description of a reaction whose rate depends on the product of the concentrations of TWO reactants
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Zero Order
rate of reaction does NOT depend on concentration of substrate, it instead relies on the presence of catalysts
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Steady state
Condition in which the \[ES\] remains constant in spite of continuous turnover
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KM – Michaelis constant
describes the affinity of enzyme for substrate
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Vmax
maximum velocity of reaction
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When \[S\] << KM
• First-order kinetics \n • Double \[S\], Vinit doubles
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When \[S\] = KM
Vinit = Vmax/2
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When \[S\] >> KM
Vinit= Vmax \n • enzyme is saturated with substrate \n • Zero-order kinetics
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Inhibitors
substances that decrease the rate of an enzyme-catalyzed reaction
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Reversible Inhibitors
Substances that bind to an enzyme and subsequently are released
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Irreversible inhibitors
Substances that react with enzymes to produce proteins that are not enzymatically active and from which original enzymes cannot be regenerated
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Competitive Inhibition
Decrease in enzymatic activity caused by binding of a substrate analogue to the active (catalytic) site DOESNT AFFECT CATALYSIS
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In the presence of a competitive inhibitor:
* Vmax is unchanged
* apparent increase in KM

\
Because substrate and inhibitor are competing for the same location, a sufficiently high amount of substrate will outcompete inhibitor
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Noncompetitive Inhibitor
Form of enzyme inactivation in which a substance binds to a site other than the active site but distorts the active site to inhibit a reaction

\
Involves two distinct binding sites: one for the substrate and one for the inhibitor
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Increasing substrate concentration cannot
overcome noncompetitive inhibition causing the Vmax to decrease and the value of kM to stay unchanged
53
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A biologist working in a lab adds a compound to a solution that contains an enzyme and its substrate. This compound binds to the enzyme and decreases the rate at which the enzyme converts substrate to product. However, this decrease can be overcome by increasing the concentration of substrate in the reaction mix. Therefore, which of the statements is true of the compound?
The compound is a competitive inhibitor
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Under steady state conditions the ES …..
remains constant
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____ can be used to compare the relative affinity of an enzyme for a substrate if the Vmax values for the enzymes are similar.
KM
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Regulation of Enzyme Activity can
Alter enzyme concentration \n

\-rate of synthesis \n -rate of degradation \n

Alter enzyme activity \n

\-allostery (activation, inhibition) \n -phosphorylation \n -post-translational processing \n -sequestration
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____ describes allosteric behavior
sigmoidal curve
sigmoidal curve
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Types of Inhibition
K systems

V systems

K 0.5
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K systems
Enzyme for which the substrate concentration that yields \n one-half Vmax is altered by the presence of inhibitors or activators
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V systems
Enzyme in which the presence of inhibitor/activator \n changes the maximal velocity of the enzyme but not the substrate level that yields one-half Vmax
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K0.5
Substrate level at one-half Vmax in a K system
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Homotropic effects
Allosteric effects that occur when several IDENTICAL molecules are bound to a protein
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Heterotropic effects
Allosteric effects that occur when DIFFERENT substances are bound to a protein
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Binding of O2 to hemoglobin exhibits
positive cooperativity
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Concerted Model
Description of allosteric activity in which the conformations of all subunits change simultaneously
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Zymogen
Inactive protein that can be activated by specific hydrolysis of peptide bonds
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Negative Cooperativity
Cooperative effect whereby binding of the first ligand \n to an enzyme or protein causes the affinity for the next ligand to be lower
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Transition-state analog
Synthesized compounds that mimic the form \n of the transition state of an enzyme reaction
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Abzymes
Antibodies that are produced against a transition-state analog and that have catalytic activity similar to that of a naturally occurring enzyme
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Coenzymes
Nonprotein substances that take part in enzymatic reactions and are regenerated at the end of the reaction
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Amphipathic
Molecule that has one end with a polar or charged, water-soluble group and another end with a nonpolar hydrocarbon group that is insoluble in water
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Fatty Acids
Unbranched chain carboxylic acids

* Amphipathic compounds
* 12–20 carbons long
* Unsaturated fatty acids - Contain \n carbon–carbon double bonds
* Saturated fatty acids - Contain only \n single bonds
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Triacylglycerols
Lipids formed by the esterification of \n three fatty acids to glycerol

\
Ester groups form the polar part of the molecule, and the tails are nonpolar
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Phosphatidic Acid
Compound in which two fatty acids and phosphoric acid are esterified to the three hydroxyl groups of glycerol
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Phosphoacylglycerols
•Polar head group is charged \n

•Phosphate group is ionized at \n neutral pH \n

•Positively charged amino group is \n contributed by an amino alcohol \n esterified to the phosphoric acid
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Waxes
•Complex mixtures of esters of long-chain carboxylic acids and long- \n chain alcohols \n

•Serve as protective coatings for plants and animals
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Sphingolipids
•Contain sphingosine, a long- \n chain amino alcohol \n

•Found in plants and animals and \n are abundant in the nervous \n system
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Glycolipids
•Lipid to which a sugar moiety is bonded
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Steroids
•Lipids with a characteristic fused-ring structure •Three six-membered rings (the A, B, and C rings) \n •One five-membered ring (the D ring)

\
Four fused rings in total
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What are important steroids?
Cholesterol is an important steroid, it acts as a precursor for other steroids
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Interaction between lipid bilayers and membrane proteins determines….
membrane function
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Lipid Bilayer
Aggregate of a lipid molecule in which the polar head groups are in contact with water and the hydrophobic parts are not that is held together by noncovalent interactions (van der waals and hydrophobic interactions)
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When considering the Lipid Bilayer and cholesterol it is important to remember …….?
That the hydrocarbon interior of saturated and unsaturated fatty acid chains and the fused-ring system of cholesterol
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Which of the following lipids is not \n found in biological membranes?
triacylglycerols
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What is important regarding the OUTER layer of the lipid bilayer?
Sphingolipids, gangliosides, cerebrosides have larger hydrophilic groups and are more likely to be found on the outer layer of a curved bilayer
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What is important regarding the INNER layer of the lipid bilayer?
phosphoacylglycerols with smaller hydrophilic groups are more common on the inner layer.
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The most abundant open-chain lipids found in typical membranes are
phosphoacylglycerols such as phosphatidylethanolamine and phosphatidylcholine
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Membrane fluidity can differ due to the composition of the bilayer for instance …..
Saturated fatty acids have a LINEAR arrangement of hydrocarbon chains leading rigidity

\
Unsaturated fatty acids have a KINKY arrangement that causes disorder in its packing and leads to greater fluidity
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With membrane fluidity and cholesterol …….
The presence of cholesterol can enhance order and rigidity due to the fused ring structure of cholesterol
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Receptor proteins in the membrane ….
detect extracellular signals and trigger intracellular signaling pathway
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Peripheral proteins are …
Loosely bound to the outside of a membrane