Chapter 4 Enzymes and Energy

Which of the following correctly represents the mechanism of enzyme function (S = substrate, P = product, E = enzyme)?

E + S -> E - S -> E - P -> E + P

An enzyme speeds up a chemical reaction in the cell, but can only be used once. TRUE or FALSE

FALSE

Blood sugar refers to what molecule circulating in blood?

Glucose

Slightly different forms of an enzyme found in different organs are called _________.

Isoenzymes

How often can a sucrase molecule be used to hydrolyze sucrose?

Many times

____ metabolic pathways are branched.

Most

Where is sucrase found in the human body?

On the microvilli of the small intestine

The active site is a binding site for which of the following?

Substrate

How does sucrose change the configuration of sucrase?

Sucrose binds to its active site on sucrase, changing the shape of sucrase enzyme.

Lingual lipase is an enzyme with a pH optimum near 5. The pH optimum of an enzyme usually reflects the pH of the body fluid in which the enzyme is found. Thus, this enzyme's pH optimum is lower most enzymes in the body. TRUE or FALSE

TRUE The optimum pH of most enzymes in the body is between 6 and 8. Recall that normal blood pH is 7.35 to 7.45.

Coenzymes NAD and FAD are found in the vitamins niacin and riboflavin. Why is it that excess amounts of these vitamins are removed from the body in the urine?

They are water-soluble vitamins and therefore not stored in the body.

The term oxidation does not mean that oxygen has to be involved. Rather it arises from the fact that oxygen readily _________ electrons and therefore acts as a strong __________ agent.

accepts; oxidizing

The amount of energy required for a reaction to occur is called the __________________________.

activation energy

End-product inhibition acts on the _______ enzyme.

branch-point

The function of sucrase is to ________.

break sucrose into glucose and fructose

It is false that most reactions in the body occur without the need for _______.

catalysts

Cofactor ________ the active site of the enzyme as well as participates in the bonding between the enzyme and the ________

changes; substrates

Oxidation-reduction reactions always occur in _________ reactions, because an atom or molecule cannot become _________ unless there is a molecule to which it can donate its electrons.

coupled; oxidized

Catalysts _____ the heat of a reaction.

do not change

Reactions that release energy are called __________ reactions.

exergonic Correct

It is _______ that most reactions in the body occur without the need for catalysts.

false

In the pathway shown, if the product F exhibited allosteric inhibition, then the pathway leading to product F' would be _______.

favored

Oxidation-reduction reactions in cells often involve the transfer of ____________ atoms rather than free ____________. A hydrogen atom contains 1 electron and 1 proton; therefore a molecule that loses hydrogen becomes ____________, and one that gains hydrogen becomes ____________. NAD is an example of a ____________ that transfers pairs of ____________ atoms from one molecule to another. In the reaction shown on the left, NAD is ____________ (it acts as a oxidizing agent); whereas in the reaction on the right, NAD is ____________ (it acts as a reducing agent).

hydrogen, electrons; oxidized, reduced; coenzyme, hydrogen, reduced, oxidized

As temperature increases enzyme activity ____________ up to a point, a characteristic ____________ , after which activity ____________ . Looking at the graph, enzyme activity peaks at around ____________ C and is lowest around ____________ C. The peak corresponds to ____________ temperature and the decrease reflects an altered ____________ structure of enzymes at ____________ temperatures.

increases, plateau, decreases; 40, 0, body, tertiary, higher

Consider the following reversible reaction: CO2 + H2O <--> H2CO3. In which direction (right or left) will the reaction proceed if the CO2 concentration in the blood is reduced?

left

Catalysts ______ the activation energy needed for a reaction to occur.

lower

Reversible reactions are governed by the law of ____________, the principle that the reaction will be driven from the side of the equation where the concentration is ____________ to the side where it is ____________. In the tissues, where CO2 is high, the reaction will move to the ____________. In the lungs where CO2 is low and H2CO3 is high, the reaction will move to the ____________. In other words, the direction of the reaction depends on the ____________ of the molecules on each side of the equation.

mass action, high, low; right, left, relative concentrations

The figure above shows a series of enzymatic reactions known as a ____________ pathway. As in this example, most metabolic pathways are ____________ rather than linear. The activities of enzymes at branch points are often regulated by ____________ inhibition, where the ____________ product of one pathway inhibits the ____________ enzyme in that pathway. This inhibition prevents the over production of the product of that pathway and shifts production to the ____________ pathway. The mechanism by which the product inhibits the enzyme is known as ____________ inhibition. Binding of the inhibitor to the enzyme causes the ____________ of the enzyme to change shape so that it can no longer properly bind ____________.

metabolic pathway, branching, end-product, final, first, alternate, allosteric, active site, substrate.

Cofactors include ________.

metal ions

In a branched metabolic pathway, the product of an intermediate becomes the substrate for _____ enzyme.

more than one

Enzymes exhibit peak activity in a ____________ pH range called the ____________ . If the pH is not in this range the reaction rate will ____________ . The pH optimum of an enzyme corresponds to the ____________ in which it is found. For instance ____________ has a pH optimum around ____________ to allow it to work in the extremely ____________ gastric juice; whereas trypsin has a pH optimum around ____________ to allow it to work in the ____________ pH of the pancreatic juice.

narrow, pH optimum, decrease, body fluid; pepsin, 2; 9, alkaline

When an atom or molecule loses electrons it is said to be ___________.

oxidized

Enzymes are a subclass of ____________.

proteins

When an atom or molecule gains electrons it is said to be ___________.

reduced

The atom or molecule that donates electrons is known as a _________ agent and the one that accepts electrons is the __________ agent.

reducing; oxidizing

The direction of reversible reactions depends on the __________ concentrations of the reactants and products.

relative

Some enzymatic reactions can move both forward and backward, which means they are ____________. For example, the enzyme carbonic anhydrase catalyzes the following reaction: H₂O + CO₂ ↔ H₂CO₃

reversible

In a metabolic pathway, the product of one enzyme subsequently becomes the ____ of the next.

substrate

It is ________ that many enzymes are inactive without the presence of cofactors or coenzymes.

true