Enzyme Overview and Nomenclature

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Questions and Answers

Which characteristic is NOT typically associated with enzymes?

They can be denatured by alterations in pH or heat.

They lower the activation energy required for reactions.

They are composed of specific amino acid sequences.

They are consumed during the biochemical reaction. (correct)

According to the Enzyme Commission (EC), what does the 'systematic name' of an enzyme define?

The abbreviated term widely used in laboratories.

The substrate acted upon, the reaction catalyzed, and any coenzymes. (correct)

A practical and easy name for laboratory use.

A four digit code and classification of the enzyme.

What action is characteristic of a lyase?

Catalyzing hydrolysis of various bonds

Catalyzing the transfer of a group from one substrate to another

Catalyzing oxidation-reduction reactions

Catalyzing the removal of groups from substrates, resulting in double bonds (correct)

Which enzyme class facilitates the conversion of geometric, optical, or positional isomers?

Isomerases (D) Signup and view all the answers

If an enzyme catalyzes the addition of water to a substrate, what class of enzyme does it belong to?

Hydrolases (D) Signup and view all the answers

Under zero-order kinetics, what is the primary factor determining the velocity of an enzymatic reaction?

The concentration of the enzyme. (D) Signup and view all the answers

An enzymatic reaction is proceeding under conditions where an increase in substrate concentration does not lead to a corresponding increase in reaction rate. Which condition is most likely causing this?

The enzyme is under saturated conditions. (D) Signup and view all the answers

In an enzyme assay designed to measure enzyme concentration, why is it crucial to have a substrate concentration significantly higher than the Km?

To ensure that the substrate does not deplete during the assay. (A) Signup and view all the answers

An enzymatic reaction, initially at 25°C, has its temperature raised to 45°C. Assuming a Q10 of 2, what would be a reasonable expectation of the change in the reaction rate?

The reaction rate will approximately quadruple. (D) Signup and view all the answers

Which of the options represents a typical example of a cofactor that directly activates an enzyme?

Magnesium ion ($Mg^{2+}$). (D) Signup and view all the answers

Which statement accurately describes the function of ligases?

They catalyze the joining of two substrate molecules utilizing the energy from ATP hydrolysis. (B) Signup and view all the answers

Why might increased levels of enzymes in the blood indicate a disease process?

Enzyme levels are normally low, so increased levels suggest cellular damage or leakage. (C) Signup and view all the answers

During which phase of an enzyme-catalyzed reaction does product formation follow zero-order kinetics, allowing for accurate measurements?

Linear Phase, when product formation and substrate consumption are consistent. (B) Signup and view all the answers

What is a primary limitation of using a 2-point (fixed-time) assay for measuring enzyme activity?

The readings may not be taken during the linear phase of the enzyme reaction. (B) Signup and view all the answers

Which method for reading enzyme reactions is more accurate for measuring enzyme activity due to its detection of deviations from linearity?

Kinetic Assay (Continuous Monitoring) because provides multiple absorbance readings. (D) Signup and view all the answers

Which term describes an enzyme that is structurally inactive and requires modification by another enzyme to become active?

Proenzyme/Zymogen (C) Signup and view all the answers

In enzyme kinetics, under what conditions is the rate of reaction directly proportional to the enzyme concentration?

Zero-order conditions with the substrate in excess (D) Signup and view all the answers

What is the primary difference between a coenzyme and a prosthetic group?

A coenzyme is loosely bound, while a prosthetic group is tightly bound. (B) Signup and view all the answers

Which of the following is TRUE when an enzyme is used as a reagent to measure a non-enzymatic analyte?

The reaction rate is proportional to the substrate concentration. (C) Signup and view all the answers

An enzyme found in serum is measured to have activity of 250 IU/L. How would this measurement be interpreted?

The enzyme will catalyze the reaction of 250 μmol of substrate in 1 liter per minute. (A) Signup and view all the answers

Study Notes

Enzyme Overview

Enzymes are specific proteins that catalyze biochemical reactions.
They lower the activation energy needed for reactions to proceed.
Enzymes are not consumed or altered during the reaction.
They are regenerated and act on a substrate.
They can be found in all body tissues.
Enzyme levels in the serum can increase after cellular injury.
Enzymes are proteins composed of specific amino acid sequences.
They can be denatured by heat and changes in pH.
Isoenzymes and isoforms are different forms of the same enzyme.

Enzyme Nomenclature

The Enzyme Commission (EC) developed a classification system for enzymes in 1961.
Systematic names describe the substrate, reaction, and coenzyme.
Recommended names are more practical.
Abbreviations are used in laboratories.
Code numbers are four-digit numbers separated by decimal points.
The first digit places the enzyme into one of six classes.

Enzyme Classes

Oxidoreductases catalyze oxidation-reduction reactions between two substrates.
Transferases transfer a group other than hydrogen from one substrate to another.
Hydrolases catalyze hydrolysis of various bonds.
Lyases remove groups from substrates without hydrolysis.
Isomerases interconvert geometric, optical, or positional isomers.
Ligases catalyze the joining of two substrate molecules.

Clinically Significant Enzymes

Lactate dehydrogenase (LDH)
Aspartate aminotransferase (AST)
Alanine aminotransferase (ALT)
Creatine kinase (CK)
Gamma glutamyltransferase (GGT)
Alkaline phosphatase (ALP)
Acid phosphatase (ACP)
Amylase (AMS/AMY)

Enzyme Activity Measurement

Measuring enzyme activity is common and relates to enzyme concentration.
Enzyme concentrations in blood are normally low.
Increased levels indicate disease.
Immunoassays can detect some enzymes directly.
Enzyme concentration can be measured by mass (e.g., Creatine Kinase, CK).
Electrophoresis is used to measure isoenzymes or isoforms.
Different methods for measuring enzyme activity exist.

Enzyme Kinetics

Enzyme activity can be measured by changes in product concentration, substrate concentration or coenzyme concentration.
The initial phase of the reaction may exhibit zero-order kinetics.
Zero-order kinetics depend only on enzyme concentration, not on substrate concentration.
If other variables affect the rate (e.g., substrate depletion), then First-order kinetics exist.
Measurements are done to specific time points and are either 2-point (or fixed time) assays or kinetic assays.
Enzyme concentration can be very high in a sample. If this occurs the reaction won't be linear, the sample can be diluted and reanalyzed.

Michaelis-Menton Curve

Km is the Michaelis constant (substrate concentration at half maximum velocity).
Max velocity is the maximum rate of reaction.
Vmax is the maximum reaction velocity.
Manufacturers will ensure that substrate concentration is greater than Km so zero order kinetics apply to the assay.

Factors Influencing Enzyme Activity

Substrate concentration ensures that the substrate does not run out.
Reaction should be buffered at an optimal pH.
Changes in pH can denature enzymes.
Physiologic enzymatic reactions occur between pH 7.0 and 8.0.
Temperature affects enzyme activity.
Increased temperature increases activity until too high leading to denaturation.
Rate of reaction doubles with every 10°C increase (Q10).
Enzyme concentration directly relates to reaction speed (higher concentration = faster reaction).

Cofactors

Cofactors are nonprotein molecules that bind to enzymes.
They are required for some enzymes to function.
Inorganic cofactors include metals (e.g., Ca2+, Fe2+, Mg2+) and nonmetals (e.g., Br, Cl-).
Organic cofactors include coenzymes (phosphates and vitamins).

Inhibitors & Activators

Activators increase the rate of reaction.
Inhibitors decrease the rate of reaction.

Enzyme Terms

Coenzyme: an organic cofactor.
Prosthetic group is a tightly bound coenzyme.
Holoenzyme is an active enzyme with all required components.
Apoenzyme is the protein portion of an enzyme without any cofactors.
Proenzyme (zymogen): an inactive enzyme form.
The inactive form of an enzyme that can be converted into active enzyme through specific chemical processes, mainly in digestive enzymes.

Enzyme Reporting (Activity)

The international unit (IU) is used to report enzyme activity.
1 IU is the amount of enzyme needed to catalyze the reaction of 1 µmol of substrate per minute per liter.
The results are reported at IU/L or mIU/mL..

Enzymes as Reagents

Enzymes are used to measure non-enzymatic substances.
Glucose oxidase is used for glucose measurement, and uricase measures uric acid.

Clinical Significance & Sources of Error

Enzymes produced in cells can be found in plasma or fluids.
Increased enzymes can indicate disease, (ex. increased production, or leakage) of the tissue or organ.
Hemolyzed samples should be avoided since enzymes are higher in red blood cells.
Measurements should be made ASAP to prevent an increase due to standing.
Increased triglycerides are a source of error, which can falsely diminish enzyme measurement values.
Some enzymes, such as acid phosphatase, are unstable at room temperature and must be measured quickly.

Isoenzymes

Isoenzymes are different forms of the same enzyme.
They have the same active site, catalyzing the same reaction.
They differ in their amino acid structure.
Isoenzymes can come from different tissues, such as Liver, bone or organs.
Electrophoresis and immunochemical methods can be used for identification.

Methods to Identify Isoenzymes

Zone electrophoresis (AKA protein electrophoresis) is used, Enzymes have different protein structures.
Selective inactivation of enzymes allows specific isoenzymes to be examined.
Increased temperature or chemical inactivation will denote specific isoenzyme from other inactive isoforms.
Immunochemical methods use antisera to identify specific isoenzymes.

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Description

Explore the essentials of enzyme functionality, classification, and nomenclature. This quiz covers key concepts such as their role as catalysts, structural characteristics, and the classification system set by the Enzyme Commission. Test your understanding of enzymes and their significance in biological reactions.