chapter 13 section 5

Question 1

What are bisubstrate reactions?

Answer 1

Enzyme-catalyzed reactions involving two substrates that yield two products.

Question 2

What are the two main types of bisubstrate mechanisms?

Answer 2

Sequential (single-displacement) and ping-pong (double-displacement).

Question 3

What defines sequential reactions?

Answer 3

Both substrates bind to the enzyme before products are released.

Question 4

What are the two classes of sequential reactions?

Answer 4

Ordered (one substrate must bind first) and random (either substrate can bind first).

Question 5

What is the Lineweaver–Burk pattern for sequential reactions?

Answer 5

intersecting lines.

Question 6

What defines ping-pong (double-displacement) reactions?

Answer 6

One substrate modifies the enzyme, releasing a product; the second substrate then reacts with the modified enzyme to release the second product.

Question 7

What is the Lineweaver–Burk pattern for ping-pong reactions?

Answer 7

parallel lines

Question 8

What key feature distinguishes ping-pong reactions?

Answer 8

Formation of a modified enzyme intermediate.

Question 9

How was Viagra originally designed?

Answer 9

As a selective inhibitor of PDE 5 to treat angina or hypertension

Question 10

Why does Viagra cause erections instead?

Answer 10

By blocking PDE 5, it increases cGMP in penile tissue, relaxing smooth muscle and improving blood flow.

Question 11

What is the rate-limiting step in random, single-displacement reactions?

Answer 11

Conversion of the ternary complex (AEB → PEQ).

Question 12

In random, single-displacement reactions, does the binding order of substrates or release order of products matter?

Answer 12

No, either substrate can bind first and either product can be released first.

Question 13

How can random single-displacement reactions be distinguished from ordered ones?

Answer 13

If binding of one substrate does not affect binding of the other, Lineweaver–Burk plots intersect on the 1/[A] axis.

Question 14

What is a classic example of a random, single-displacement enzyme?

Answer 14

Creatine kinase.

Question 15

What reaction does creatine kinase catalyze?

Answer 15

Transfer of a phosphate from ATP to creatine, forming creatine phosphate (CrP).

Question 16

What role does creatine phosphate play in muscle cells?

Answer 16

Acts as a reservoir of high-energy phosphate bonds for quick ATP regeneration.

Question 17

: Does creatine kinase involve a modified enzyme intermediate (E′)?

Answer 17

No, it forms reversible binary complexes and proceeds through a ternary complex.

Question 18

What defines an ordered, single-displacement reaction?

Answer 18

The leading substrate (A) must bind first before the second substrate (B).

Question 19

Can B bind to the enzyme before A in ordered reactions?

Answer 19

No, B cannot bind without A already bound.

Question 20

What usually happens after A and B react in the ternary complex?

Answer 20

Products are released in a defined order, with P (from A) released last.

Question 21

In ordered mechanisms, which molecules are competitive for the same binding site?

Answer 21

A and P

Question 22

Which class of enzymes commonly follows ordered single-displacement kinetics?

Answer 22

NAD⁺-dependent dehydrogenases.

Question 23

In NAD⁺-dependent dehydrogenases, what is the leading substrate?

Answer 23

NAD⁺.

Question 24

What example illustrates this mechanism?

Answer 24

Alcohol dehydrogenase: NAD⁺ binds first, then ethanol, forming NADH and acetaldehyde.

Question 25

How do we confirm the mechanism is ordered and not random?

Answer 25

Ethanol (B) does not bind the enzyme unless NAD⁺ (A) is already bound.

Question 26

What is a defining feature of double-displacement (ping-pong) reactions in Lineweaver–Burk plots?

Answer 26

Parallel lines when 1/v is plotted versus 1/[A] at different [B].

Question 27

How does the enzyme behave in a ping-pong mechanism?

Answer 27

The enzyme is temporarily converted to a modified form (E′) after reacting with the first substrate.

Question 28

In a ping-pong reaction, which substrate reacts with the modified enzyme (E′)?

Answer 28

The second substrate, B, reacts with E′ to form the second product and regenerate E.

Question 29

Do A and Q bind to E′?

Answer 29

No, A and Q bind only to the free enzyme (E).

Question 30

Do B and P bind to E?

Answer 30

No, B and P bind only to the modified enzyme (E′).

Question 31

What is an example of enzymes that use a ping-pong mechanism?

Answer 31

Aminotransferases (transaminases).

Question 32

How do aminotransferases catalyze reactions?

Answer 32

They transfer an amino group from an amino acid to an α-keto acid, forming a new amino acid and keto acid

Question 33

What coenzyme is involved in glutamate:aspartate aminotransferase?

Answer 33

Pyridoxal phosphate (vitamin B6 derivative), which cycles between pyridoxal (in E) and pyridoxamine phosphate (in E′).

Question 34

What is one diagnostic method to distinguish bisubstrate mechanisms?

Answer 34

Graphic analysis of kinetic patterns and competitive effects between substrates.

Question 35

Which enzyme can catalyze a phosphate exchange into glucose-1-phosphate?

Answer 35

Sucrose phosphorylase.

Question 36

What is an exchange reaction in enzyme kinetics?

Answer 36

A double-displacement reaction where a chemical group transferred from a substrate to the enzyme is transferred back to a different molecule of the same substrate.

Question 37

Can maltose phosphorylase catalyze a similar phosphate exchange?

Answer 37

No, it requires formation of a ternary complex and follows a single-displacement mechanism.

Question 38

What feature distinguishes enzymes that follow double-displacement mechanisms?

Answer 38

they can catalyze exchange reactions even with only one substrate and the modified enzyme intermediate present.

Question 39

In sucrose phosphorylase, what is the enzyme intermediate during the double-displacement mechanism?

Answer 39

E-glucose.

Question 41

How is radioactive 𝑃 32 P 32 used in studying exchange reactions

Answer 41

P32 incorporation into glucose-1-phosphate shows the enzyme transfers phosphate via E′ in a double-displacement reaction.

Question 42

Can enzyme-catalyzed reactions involve more than two substrates?

Answer 42

Yes; many enzymes in metabolism act on three or more substrates.

Question 43

Give an example of a multisubstrate enzyme.

Answer 43

Glyceraldehyde-3-phosphate dehydrogenase: NAD⁺ + glyceraldehyde-3-P + Pi ⇌ NADH + H⁺ + 1,3-bisphosphoglycerate

Question 44

How are multisubstrate reactions analyzed kinetically?

Answer 44

By breaking them down into a series of unisubstrate or bisubstrate steps.

Question 45

By breaking them down into a series of unisubstrate or bisubstrate steps.

Answer 45

Yes, each step obeys single- or double-displacement (ping-pong) mechanisms.