Ch.6 Energy, Enzymes, & Biological Reactions

The first law of thermodynamics states that the total disorder of a system always increases.

False – second law of thermodynamics

The potential energy in a chemical bond is defined as chemical energy.

True

At equilibrium, the concentrations of the reactants equals that of the products.

False – rate of formation

Reactions that reach an equilibrium point are reversible.

True

ATP synthesis is exergonic.

False – endergonic

Enzymes alter the equilibrium point of a reaction.

False – activation energy

Enzymes don't change the DG of a reaction.

True

The rate of catalysis is proportional to the concentration of the enzyme.

True

If an enzyme is bound by an allosteric activator, the enzyme will convert from a low-affinity state to a high-affinity state.

True

Ribozymes are enzymes.

False – biological catalysts

The term that best describes all of the chemical reactions of a cell, including acquisition and use of molecules and energy, is ____

metabolism

The removal of a phosphate group during an enzyme-catalyzed reaction takes ____

about 10 milliseconds

A child swinging on a swing utilizes which type(s) of energy?

kinetic, potential, and chemical energy: the child powers the swing with chemical energy in the muscle, cells and the swing moves like a pendulum with changing ratios of kinetic and potential energy.

The ultimate fate of the energy used by organisms is ____

conversion into heat

In molecules, the constant motion of the atoms is an example of ____ energy, while the arrangement of atoms and bonds is an example of ____ energy.

kinetic; potential

Which system is considered a closed system?

the Earth

Which system is considered an open system?

a human

According to the first law of thermodynamics, ____

energy only changes forms

Which statement is a part of the first law of thermodynamics?

Energy can be neither created nor destroyed.

What is the ultimate source of energy for almost all organisms?

the sun

We can calculate whether a reaction is spontaneous by calculating the change in free energy and accounting for entropy. Your paycheck always lists your gross pay, net (take home) pay, and tax withholdings. Which of the following best correlates your paycheck to the changes in free energy?

net salary = gross salary - tax; free energy = total energy - entropy

During every energy transformation, it can be said that ____

the entropy of the universe increases

Which example would have a negative change in entropy?

water freezing

Which equation is used to calculate the free energy associated with a reaction?

ΔG = ΔH - TΔS

Although energy cannot be created or destroyed, energy transformations are not 100% efficient. Why?

Energy is lost as an increase in entropy.

If a reaction is endergonic, what can we infer about the reaction?

ΔG must be positive

Which two factors determine whether a reaction is spontaneous?

enthalpy and entropy

If a reaction is spontaneous, then ΔG is ____ and the reaction is ____

negative; exergonic

What can be inferred from the accompanying graph?

This reaction is endergonic, with a positive ΔG.

In an exothermic reaction, ____

the products have less potential energy than the reactants

Reversible reactions in a cell rarely reach equilibrium because ____

the products are generally reactants in other reactions and are thus immediately used

When an enzyme-catalyzed reaction reaches equilibrium ____

the rate of the forward and reverse reactions are equal

Identify the exergonic reaction in the list below.

burning wood for a campfire

Which statement is true for exergonic reactions?

The products have less free energy than the reactants.

Which reaction is most likely to have more products than reactants when it reaches equilibrium?

DG = -100 kcal/mol

An exergonic reaction will have a ____

negative change in free energy

Eating and digesting a candy bar for energy during a sports event is a good example of ____

catabolism

The breakdown of glucose into carbon dioxide, water, and ATP is an example of a(n) ____ pathway.

catabolic

Energy from ATP is transferred to the reactant of an endergonic reaction by addition of a(n) _ group to that molecule.

phosphate

The free energy of ATP hydrolysis is -31.0 kJ/mol. The free energy of glucose phosphorylation by hexokinase is

14.3 kJ/mol. These two reactions are coupled to allow them to proceed spontaneously. What is the overall free energy associated with this coupled reaction?

How does energy coupling allow chemical reactions that are not spontaneous to proceed?

the energy from the exergonic reaction is transferred to the substrate to destabilize it

How do cells overcome the energy requirement of endergonic reactions?

coupling endergonic and exergonic reactions

Where does the energy for ATP synthesis come from?

catabolism of complex molecules into simpler molecules

Approximately how many ATP molecules are hydrolyzed and resynthesized each second in a typical cell?

10,000,000

Answer the question using the accompanying graph. Which portion of the graph shows the activation energy in the absence of enzyme?

A

Answer the question using the accompanying graph. Which portion of the graph shows the activation energy in the presence of enzyme?

B

Answer the question using the accompanying graph. Which portion of the graph shows the free energy of the reaction?

C

The conversion of a diamond into graphite is a spontaneous reaction. Why are most women walking around with diamond rings and not graphite rings?

the energy of activation is very high

Enzymes aid in metabolism by ____

stabilizing the transition state

Enzymes ____

change the rate of a reaction

What is the primary determinant of the function and specificity of an enzyme?

the enzyme's conformation

Enzymes function primarily by ____

increasing the probability that the reactants will come into close proximity to each other in the proper orientation for forming the transition state molecule

Enzymes are ____ catalysts.

protein

You modify the primary sequence of an enzyme in a region that will be the active site when the protein is properly folded. What is the predicted outcome of this change?

The enzyme will not bind to the substrate properly.

What is the difference between cofactors and coenzymes?

cofactors can be inorganic or organic, coenzymes are organic cofactors

What is the name of the specific region of an enzyme responsible for catalysis?

active site

Coenzymes that bind tightly to enzymes are called ____

prosthetic groups

What happens to an enzyme after it has catalyzed a reaction?

it returns to its original state

What is the purpose of ionic groups in the active sites of enzymes?

to alter the substrate in a way that favors catalysis

If an enzyme is saturated, ____

the reaction is being catalyzed at the maximum rate

You do an experiment adding increasing substrate to an enzyme at optimal pH and temperature. What will the rate vs. substrate graph look like over time?

The rate increases rapidly, tapers off, and plateaus.

In competitive inhibition, the ____

inhibitor binds to and directly blocks the active site of the enzyme

How does the cell overcome inhibition from irreversible inhibitors?

by degrading the enzyme-inhibitor complex and generating new enzyme

To prevent hair loss, inhibitors similar to testosterone block conversion to DHT. How is this inhibition classified?

competitive inhibition

Allosteric inhibitors are often ____

products of the reactions that they regulate

What happens when an enzyme is bound by an allosteric activator?

The enzyme transitions from a low affinity state to a high affinity state.

In the accompanying temperature figure, why does the curve sharply drop after ~45°C?

The enzyme begins to denature above a certain temperature, eliminating all catalytic activity of the protein.

If an enzyme's optimal temperature is 37°C, then the enzyme ____

activity will drop at temperatures above 37°C and likely be eliminated by 60°C

Answer the question using the accompanying pH graph. The optimal pH for enzyme 1 is ____

3

Answer the question using the accompanying pH graph. The optimal pH for enzyme 2 is ____

7

If all three enzymes catalyze the same reaction and you require pH 8, which enzyme would you choose?

enzyme 3

Why do Siamese cats have brown ears, nose, tails and paws?

Siamese cats contain heat sensitive enzymes that are more active in the extremities.

What is the optimum pH of most enzymes?

7

Is a ribozyme a true enzyme?

No, because it is not a protein.

Zhang and Cech’s experiments confirmed which feature of ribozyme activity?

Ribozymes catalyze formation of bonds between amino acids in protein synthesis.

Ribozymes are ____

RNA catalysts

Harry Noller's ribosome experiments had one major flaw. What was the flaw?

undetectable amounts of protein may have remained

How did Zhang and Cech purify the RNA molecules that catalyze linkage between two amino acids?

passing the reaction mixture through a column that binds biotin

The discovery of ____ suggested that nucleic acids likely existed prior to proteins.

ribozymes

Because of problems explaining how the first RNA organisms originated, an alternative proposal suggests ____

a different form of life existed before the RNA world

coupled reaction

The linking of an exergonic reaction with an endergonic reaction that allows a cell to drive a nonspontaneous reaction to completion

equilibrium point

A state in which the rate of the forward reaction equals the rate of the reverse reaction

metabolic pathway

A series of chemical reactions where the products of one reaction are the reactants for a subsequent reaction

catalyst

A substance that facilitates a chemical reaction without itself being consumed by the reaction

activation energy

The energy needed to start a reaction, be it endergonic or exergonic

active site

The portion of the enzyme that binds to a reactant or reactants

substrate

The reactant molecule that binds to an enzyme

transition state

An intermediate arrangement of unstable bonds that can proceed toward reactants or products

allosteric regulation

The product of the reaction interacts with an enzyme in a noncompetitive way to inhibit or enhance enzyme activity

protein synthesis

endergonic

digestion of a candy bar

exergonic

a dead cell

equilibrium

a reaction where DG is positive

endergonic

a reaction where DG is negative

exergonic

the rate of synthesis equals the rate of degradation

equilibrium

Why is energy defined as "the capacity to do work"?

Energy can only be detected through its effects on matter—moving objects against forces or pushing reactions toward completion.