3.5 Enzymes 1

Question 1 What is an enzyme?

A catalyst that increases the rate of a chemical reaction without being consumed or permanently changed in the overall process.

Question 2 What are most enzymes chemically?

Proteins.

Question 3 What is the molecule on which an enzyme acts called?

The substrate.

Question 4 What general role do enzymes have in metabolism?

They selectively channel substrates into useful pathways and direct metabolic events.

Question 5 Are enzymes consumed during the reactions they catalyze?

No. They are regenerated and can participate in additional catalytic cycles.

Question 6 What rare nonprotein molecules can have catalytic activity?

Certain RNA molecules called ribozymes.

Question 7 What types of bonds can ribozymes affect according to the sources?

Phosphodiester bonds and, in some cases, peptide bonds.

Question 8 Why are enzymes biologically necessary?

They allow reactions to proceed rapidly under cellular conditions by lowering the activation-energy barrier.

Question 9 What is the biomedical significance of enzyme leakage from injured cells?

Intracellular enzymes can enter plasma or serum and serve as markers of tissue injury.

Question 10 Why can plasma enzyme activity help identify tissue damage?

Some enzymes or isoenzymes are concentrated in particular tissues, so their elevation can indicate injury to those tissues.

Question 11 According to the lecture, when may CPK-MB appear in blood after myocardial injury?

Approximately 6-12 hours after injury.

Question 12 According to the lecture, when may SGOT appear in blood after myocardial injury?

Approximately 12-24 hours after injury.

Question 13 According to the lecture, when may LDH appear in blood after myocardial injury?

Approximately 12-36 hours after injury to the myocardium.

Question 14 What does CPK-MB or CK-MB refer to?

The MB isoenzyme of creatine kinase, which is enriched in cardiac muscle.

Question 15 What is SGOT also commonly called?

Aspartate aminotransferase, or AST.

Question 16 Why do injured cells release intracellular enzymes into the circulation?

Cell membrane damage or necrosis allows intracellular enzymes to leak into plasma.

Question 17 What is plasma?

The fluid, noncellular portion of blood.

Question 18 What is serum?

The fluid obtained after blood is allowed to clot and is then centrifuged.

Question 19 Which sample is commonly used for laboratory enzyme-activity assays?

Serum.

Question 20 What can the degree of plasma enzyme elevation indicate?

It often correlates with the extent of tissue damage and may assist in prognosis.

Question 21 Which enzyme is abundant in liver and is used as a marker of hepatic injury?

Alanine aminotransferase, or ALT.

Question 22 What are isoenzymes or isozymes?

Different molecular forms of an enzyme that catalyze the same reaction but differ in physical properties or amino acid sequence.

Question 23 Why can isoenzymes be separated by electrophoresis?

They may contain different numbers of charged amino acids and therefore have different electrophoretic mobilities.

Question 24 What are the three creatine kinase isoenzymes?

CK1 = BB, CK2 = MB, and CK3 = MM.

Question 25 Which CK isoenzyme predominates in brain?

CK-BB.

Question 26 Which CK isoenzyme predominates in skeletal muscle?

CK-MM.

Question 27 Which CK isoenzyme is especially useful in myocardial-infarction assessment?

CK-MB.

Question 28 According to Lippincott, when does CK-MB appear after acute myocardial infarction?

Approximately 4-8 hours after the onset of chest pain.

Question 29 According to Lippincott, when does CK-MB peak and return to baseline?

It peaks at about 24 hours and returns to baseline in approximately 48-72 hours.

Question 30 Which cardiac biomarkers are considered highly sensitive and specific for myocardial injury?

Cardiac troponin T and cardiac troponin I.

Question 31 What two names may be assigned to an enzyme?

A recommended name and a systematic name.

Question 32 What is a recommended enzyme name?

The commonly used name, often ending in the suffix -ase.

Question 33 How may a recommended enzyme name be based on its substrate?

The suffix -ase is attached to the substrate name, as in urease or sucrase.

Question 34 How may a recommended enzyme name describe enzyme action?

It can name the reaction performed, as in lactate dehydrogenase or adenylate cyclase.

Question 35 What are trivial enzyme names?

Traditional names that do not reveal the substrate or reaction, such as trypsin and pepsin.

Question 36 What is a systematic enzyme name?

A detailed name that describes the chemical reaction and usually identifies the substrates involved.

Question 37 Why are systematic enzyme names useful?

They are unambiguous and informative.

Question 38 Why are systematic enzyme names not always used in ordinary discussion?

They are often too cumbersome.

Question 39 Give the systematic name example used for lactate dehydrogenase.

Lactate:nicotinamide adenine dinucleotide oxidoreductase.

Question 40 What enzyme classification number is given for lactate:NAD+ oxidoreductase?

EC 1.1.1.27.

Question 41 How many major enzyme classes are listed in the uploaded sources?

Six.

Question 42 Enumerate the six major enzyme classes in order.

Oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases.

Question 43 What reactions are catalyzed by oxidoreductases?

Oxidation-reduction reactions.

Question 44 Give an example of an oxidoreductase.

Lactate dehydrogenase.

Question 45 What does lactate dehydrogenase convert in the illustrated reaction?

Lactate and NAD+ to pyruvate, NADH, and H+.

Question 46 What reactions are catalyzed by transferases?

Transfer of functional groups between donor and acceptor molecules.

Question 47 Which types of groups are specifically mentioned for transferases?

Carbon-, nitrogen-, or phosphorus-containing groups.

Question 48 Give an example of a transferase.

Serine hydroxymethyltransferase.

Question 49 What are kinases?

Transferases that transfer a phosphate group, commonly from ATP, to another molecule.

Question 50 What reactions are catalyzed by hydrolases?

Cleavage of bonds by addition of water.

Question 51 Give an example of a hydrolase.

Urease.

Question 52 What reaction is catalyzed by urease?

Hydrolysis of urea to carbon dioxide and ammonia.

Question 53 What reactions are catalyzed by lyases?

Cleavage of C-C, C-S, or certain C-N bonds without hydrolysis or oxidation, often forming double bonds, or the reverse addition to double bonds.

Question 54 Give an example of a lyase.

Pyruvate decarboxylase.

Question 55 What does pyruvate decarboxylase produce from pyruvate?

Acetaldehyde and carbon dioxide.

Question 56 What reactions are catalyzed by isomerases?

Intramolecular rearrangements, including optical or geometric isomerization.

Question 57 Give an example of an isomerase.

Methylmalonyl-CoA mutase.

Question 58 What does methylmalonyl-CoA mutase convert?

Methylmalonyl-CoA to succinyl-CoA.

Question 59 What reactions are catalyzed by ligases?

Formation of bonds between carbon and O, S, or N coupled to hydrolysis of a high-energy phosphate, usually ATP.

Question 60 Give an example of a ligase.

Pyruvate carboxylase.

Question 61 What reaction does pyruvate carboxylase catalyze in the figure?

Formation of oxaloacetate from pyruvate and carbon dioxide using ATP.

Question 62 Which enzyme class commonly adds or removes hydrogen atoms?

Oxidoreductases.

Question 63 Which enzyme class transfers functional groups between molecules?

Transferases.

Question 64 Which enzyme class uses water to split a bond?

Hydrolases.

Question 65 Which enzyme class commonly removes carbon dioxide or ammonia to form a double bond?

Lyases.

Question 66 Which enzyme class rearranges atoms within a molecule?

Isomerases.

Question 67 Which enzyme class joins two molecules using ATP energy?

Ligases.

Question 68 What is the difference between a synthetase and a synthase?

A synthetase requires ATP, whereas a synthase does not require ATP directly.

Question 69 What does a phosphatase do?

It uses water to remove a phosphate group.

Question 70 What does a phosphorylase do?

It uses inorganic phosphate to break a bond and generates a phosphorylated product.

Question 71 What is the usual electron acceptor for a dehydrogenase?

NAD+ or FAD.

Question 72 What distinguishes an oxidase from an oxygenase?

An oxidase uses oxygen as the electron acceptor without incorporating oxygen atoms into the substrate, whereas an oxygenase incorporates one or both oxygen atoms into the substrate.

Question 73 An enzyme transfers phosphate from ATP to glucose. Which class is it?

A transferase, specifically a kinase.

Question 74 An enzyme cleaves a peptide bond by adding water. Which class is it?

A hydrolase.

Question 75 An enzyme joins pyruvate and CO2 using ATP. Which class is it?

A ligase.

Question 76 An enzyme converts one stereoisomer to another. Which class is it?

An isomerase.

Question 77 An enzyme removes CO2 from a substrate without hydrolysis. Which class is it?

A lyase.

Question 78 An enzyme transfers electrons from lactate to NAD+. Which class is it?

An oxidoreductase.

Question 79 Which class includes dehydrogenases and oxidases?

Oxidoreductases.

Question 80 Which class includes most proteases, lipases, and phosphatases?

Hydrolases.

Question 81 What is an active site?

A pocket or cleft on an enzyme containing amino acid side chains involved in substrate binding and catalysis.

Question 82 How is the three-dimensional active site formed?

By folding of the enzyme protein.

Question 83 What two functional regions are identified within the active site in the lecture?

The binding site and the catalytic site.

Question 84 What is the role of the binding site?

It recognizes, binds, and orients the substrate.

Question 85 What is the role of the catalytic site?

It contains groups that facilitate bond breaking or bond formation and convert substrate to product.

Question 86 How does the binding site help catalysis?

It positions the susceptible bond of the substrate near the catalytic groups.

Question 87 What complex forms when substrate binds enzyme?

The enzyme-substrate complex, or ES complex.

Question 88 What complex may form after catalysis but before product release?

The enzyme-product complex, or EP complex.

Question 89 Write the simplified enzyme reaction sequence.

E + S

Question 90 How is substrate normally bound to an enzyme?

By noncovalent interactions.

Question 91 What is the induced-fit model?

Substrate binding causes a conformational change that makes the enzyme and substrate optimally complementary for catalysis.

Question 92 What is the lock-and-key model?

The active site is considered preformed and complementary to the substrate before binding.

Question 93 Which model better emphasizes enzyme flexibility?

The induced-fit model.

Question 94 What is catalytic efficiency in general terms?

The ability of an enzyme to accelerate conversion of substrate to product rapidly.

Question 95 How much faster are many enzyme-catalyzed reactions than uncatalyzed reactions?

Approximately 10^3 to 10^8 times faster.

Question 96 What is the turnover number?

The number of substrate molecules converted to product per enzyme molecule per second.

Question 97 What symbol is used for turnover number?

kcat.

Question 98 What typical range of kcat is given by Lippincott?

Approximately 10^2 to 10^4 per second.

Question 99 What microscopic step does kcat represent in the simple model?

The rate constant for conversion of ES to E + P.

Question 100 What is enzyme turnover in the lecture's regulatory context?

The combined processes of enzyme synthesis and degradation.