Chapter 7

Question 1

Kinetics

Answer 1

study of the rates of chemical reactions

Question 2

Consider A –> P: Velocity (V)

Answer 2

• determined by measuring how much A disappears as a function of time
• the quantity of reactant (A) that disappears in a specified unit of t
• it is equal to the appearance of product P
• Velocity of the reaction Formula: V = K[A]
• k = proportionality constant, also called rate constant

Question 3

First-order reaction

Answer 3

when velocity of a reaction is directly proportional to reactant concentration

Question 4

Second-order reaction

Answer 4

more than one substrate is present, include two reactants

Question 5

Pseudo first-order reactions

Answer 5

when there are big differences in the concentrations of the different reactants (concentration of one reactant exceeds concentration of second reactant)

Question 6

FACT: The study of reaction rates or velocity typically involves:

• Substrate concentration = the variables
• Enzyme concentration = the constant

Answer 6

• As substrate concentration changes/increases, the reaction rate also increases
• Exponential increase in reaction rate as more substrate is added, but will eventually level off b/c the enzyme concentration is held constant/not changing
• Once all enzyme molecules are occupied and engaged in the reaction, reaction rates cannot and will not change no matter how much more substrate is added

Question 7

Initial velocity (V0)

Answer 7

Initial velocity (V0) for each substrate concentration is determined from the slope of the curve at the beginning of a reaction

Question 8

Consider the following: E + S –> P

Answer 8

• Enzyme E catalyzes the conversion of substrate S into a product P
• For this reaction, we should also consider:
  E + S –> ES –> E + P
• Enzyme E combines w/ substrate S to form an ES complex w/ a rate constant k1
• ES has two possible fates: it can dissociate to E and S w/ rate constant k-1 or it can proceed to form product P w/ a rate constant of k2

Question 9

Michaelis -Mention Equation

Answer 9

describes the variation of enzyme activity as a function of concentration

1. V0 = describes the initial reaction velocity as a function of substrate concentration
2. Km = Michaelis Constants: describes enzyme substrate affinity
3. Vmax = substrate concentration when the enzyme is saturated
   - Maximal velocity possible (Vmax) can be attained when all of the enzymes (ET) is bound to substrate (S)

• At very low substrate concentrations, when the substrate concentration is lower than Km, the velocity is directly proportional to the substrate concentration
• At substrate concentration higher than Km value, the velocity is maximum and is independent of substrate concentration
• When enzyme is operating at Vmax all the available enzyme is bound
• Addition of substrate will not change the reaction rate
• Enzyme is displaying zero order kinetics (under these conditions, enzyme is said to be saturated)
• Km is equal to the substrate concentration at which the reaction velocity is half its maximum value
• Michealeis mention equation can be manipulated into one that yields a straight-line plot
• The double reciprocal equation is called the Lineweaver-Burk equation
• A plot of one over V0 vs. one over the substrate concentration is called a double reciprocal plot

Question 10

FACT: Variations in K can have physiological consequences

Question 11

FACT: Alcohol dehydrogenase converts ethanol into acetaldehyde

Question 12

FACT: acetaldehyde is processed into acetate by aldehyde dehydrogenase

Question 13

There are two different acetaldehyde dehydrogenases:

Answer 13

1. A mitochondrial form w/ low Km – can be inactive in some individuals
2. Cytoplasmic form w/ higher Km – this form is unable to process all the acetaldehyde at an adequate rate

• acetaldehyde travels into the bloodstream in some people
• These individuals will physiologically respond w/ facial flushing and rapid hear-beat – symptoms of excessive amounts of acetaldehyde in the blood

Question 14

FACT:

• For most enzymes, the Km value will lie between 10^-1 to 10^7 M
• The Km value for an enzyme will be influenced by several different factors (ex. specific substrate, pH, temp, ionic strength)

Question 15

Multiple substrate reactions are divided into two groups:

Answer 15

1. sequential reactions
2. double displacement reactions

Question 16

Allosteric enzymes

Answer 16

• catalysts and information sensors
• enzymes that have multiple active sites and distinct regulatory sites that control the flux of biochemicals through a metabolic pathway
• regulation of catalytic activity is by environmental signals
• due to regulatory properties, allow for the generation and coordination of complex metabolic pathways that involve multiple enzymes and multiple steps

 Each step does NOT need to be controlled. Only a key step needs to be controlled
 Key steps will be regulated by allosteric enzymes which subsequently provides control for the whole pathway
 The step that’s regulated is often the rate limiting step. In the example above, the conversion of A to B is the commitment step. So, enzyme one (e1) is key for regulation
 Allosteric enzymes catalyze that committed step of the complex metabolic pathways
 Michaelis-Menten enzymes will facilitate the remaining reactions and steps

 In example above, the amount of F that is synthesized can be regulated by feedback inhibition
 The step that the product F inhibits is enzyme one (e1). This inhibition will take place w/ product F binding to a regulatory site on the enzyme
 Regulatory site will be different from active site
 Regulatory site is also called allosteric site. This site can be regulated by small molecule inhibitors or small molecule activators

Question 17

FACT: sigmodial curve is based on quaternary structure of allosteric enzymes

Answer 17

Sigmoidal curve is based on the fact that all allosteric enzymes have a quaternary structure, meaning that they also have multiple actives sites and regulatory binding sites

Question 18

Sequential model

Answer 18

R state = more relaxed, enzymatically more active than T-state

T state = tense, more stable, substrate binding causes shift to R state