Enzyme Regulation and Clinical Enzymology

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Questions and Answers

What is the role of allosteric activators in enzyme regulation?

They decrease enzyme activity.

They irreversibly alter enzyme structure.

They cleave peptides to activate the enzyme.

They increase enzyme activity. (correct)

What occurs during reversible covalent regulation of enzymes?

Enzymes are permanently deactivated.

Specific groups are covalently attached to the enzyme. (correct)

Zymogens are converted into active forms.

Enzyme activity is only inhibited.

Which amino acids are typically involved in phosphorylation?

Aspartate and Glutamate

Threonine, Serine, and Tyrosine (correct)

Lysine and Methionine

Alanine and Glycine

What are zymogens?

Inactive precursor proteins that require cleavage for activation. (D) Signup and view all the answers

How do sex hormones like estrogen and testosterone regulate enzyme activity?

By inducing or repressing enzyme synthesis. (B) Signup and view all the answers

What is the physiological role of cellular enzymes in plasma during normal conditions?

They do not perform a physiological function. (C) Signup and view all the answers

Which condition can cause a transient increase in membrane permeability leading to elevated enzyme levels?

Metabolic stress without necrosis. (C) Signup and view all the answers

What type of covalent modification involves the attachment of a phosphate group?

Phosphorylation (D) Signup and view all the answers

What does a rise in serum levels of aspartate transaminase (AST) indicate?

Myocardial infarction or hepatocellular damage. (B) Signup and view all the answers

What is a consequence of covalent modification on enzymes?

The shape and activity of the enzyme may change. (D) Signup and view all the answers

What is the significant effect of proteolytic cleavage on enzyme precursors?

It causes irreversible activation of the enzyme. (C) Signup and view all the answers

Which enzyme is uniquely associated with liver function?

Alcohol dehydrogenase (A) Signup and view all the answers

In the context of clinical enzymology, what is alkaline phosphatase (ALP) primarily associated with?

Obstructive liver disease and increased osteoblastic activity. (B) Signup and view all the answers

Which enzyme is a marker for carcinoma prostate and rises in metastatic bone diseases?

Acid phosphatase (ACP) (C) Signup and view all the answers

What is a common diagnostic use of alanine transaminase (ALT) levels?

To assess parenchymal liver disease. (B) Signup and view all the answers

What can elevated levels of lactate dehydrogenase (LDH) in blood indicate?

Block in coronary blood vessel leading to heart damage. (B) Signup and view all the answers

What is the most common mechanism for the production of isoenzymes?

Different combinations of polypeptide subunits (D) Signup and view all the answers

Which characteristic of isoenzymes is NOT typically different among them?

Substrate specificity (A) Signup and view all the answers

Which isoenzyme of lactate dehydrogenase is made of all M subunits?

LDH-5 (C) Signup and view all the answers

What are allelic isoenzymes primarily characterized by?

Structural variations from the same genetic locus (C) Signup and view all the answers

What kind of changes in enzyme levels due to protein synthesis appear to be faster than isoenzyme activity changes?

Allosteric regulation (A) Signup and view all the answers

What type of modification can lead to the generation of isoforms in a single enzyme?

Post-translational modification (C) Signup and view all the answers

Which of the following is NOT a characteristic of isoenzymes?

Produced from the same genetic sequence (A) Signup and view all the answers

Which enzyme is known to exist as multiple isoenzymic forms due to different ratios of polypeptide subunits?

Lactate dehydrogenase (D) Signup and view all the answers

What does an elevated level of LDH specifically indicate in the context of myocardial infarctions?

It shows continued increase after CK and AST. (D) Signup and view all the answers

Which enzyme is primarily associated with diagnosing acute pancreatitis?

Amylase (C) Signup and view all the answers

Which condition is particularly characterized by a significant increase in alkaline phosphatase (ALP) levels?

Liver disease (D) Signup and view all the answers

In the context of prostate cancer, what does the presence of acid phosphatase indicate?

Metastatic carcinoma of the prostate (C) Signup and view all the answers

What is the primary role of lactate dehydrogenase (LDH) in cellular metabolism?

To facilitate the reverse conversion of lactate and pyruvate. (B) Signup and view all the answers

Which enzyme shows the first rise in myocardial infarctions?

Creatine Kinase (CK) (D) Signup and view all the answers

What is the significance of a 'flipped pattern' in LDH isoenzymes in myocardial infarction diagnosis?

It shows LDH2 levels decrease relative to LDH1. (D) Signup and view all the answers

Which statement accurately represents the distribution of alkaline phosphatase (ALP)?

Highest concentration in liver and bone. (D) Signup and view all the answers

What is defined as one-unit enzyme activity?

The transformation of 1.0 mmole of substrate per minute at 25°C. (B) Signup and view all the answers

Which statement best describes the turnover number of an enzyme?

It is the amount of substrate transformed per minute per enzyme molecule. (C) Signup and view all the answers

What role do enzymes play concerning activation energy?

Enzymes lower the activation energy barrier for reactions. (C) Signup and view all the answers

How is the rate of an enzyme-catalyzed reaction typically measured?

By the rate of substrate disappearance or product appearance. (A) Signup and view all the answers

Which enzyme has the highest turnover number among the given options?

Carbonic anhydrase (A) Signup and view all the answers

What happens to the change in free energy (ΔG) when an enzyme catalyzes a reaction?

It remains unchanged. (A) Signup and view all the answers

What is activation energy often supplied as?

Thermal energy from the surroundings. (D) Signup and view all the answers

Which of the following statements about enzymes is false?

Enzymes decrease the rate of the reaction. (B) Signup and view all the answers

Which isoenzyme of lactate dehydrogenase is primarily elevated in myocardial infarction?

LDH1 (C) Signup and view all the answers

What tissue primarily contributes to the production of M subunits of lactate dehydrogenase?

Skeletal muscle (B) Signup and view all the answers

Which LDH isoenzyme has the lowest percentage in serum and is associated with muscle trauma?

LDH5 (A) Signup and view all the answers

Which condition primarily results in elevated levels of LDH₄?

Muscle trauma (B) Signup and view all the answers

What combination of subunits makes up LDH₃?

HHMM (B) Signup and view all the answers

Which of the following tissues contains primarily H subunits of lactate dehydrogenase?

Heart (C) Signup and view all the answers

What is the primary function of β-hydroxybutyrate dehydrogenase?

Oxidation of β-hydroxybutyrate (B) Signup and view all the answers

Which LDH isoenzyme is mostly produced by liver tissue?

LDH4 (B) Signup and view all the answers

Study Notes

Quantitative Assay of Enzymes

Enzyme activity is measured by the rate at which a substrate is transformed into a product.
The greater the transformation rate, the higher the enzyme activity.
Enzyme activity can be estimated from the rate of product appearance or substrate disappearance.

Measurement of Enzyme Activity

One unit of enzyme activity is the amount causing the transformation of 1.0 mmol of substrate per minute at 25°C under optimal conditions.
Enzyme activity is usually expressed as mmol of substrate disappeared or mmol of product formed per minute.

Turnover Number of Enzymes

Turnover number refers to the number of substrate molecules a single enzyme molecule transforms in a specific unit of time when enzyme concentration is the rate limiting factor.
Carbonic anhydrase has a high turnover number of 36,000,000.
Examples of specific enzyme turnover numbers are listed for carbonic anhydrase, B-amylase, phosphoglucomutase, and fumarase.

Reaction Activation Energy

All chemical reactions require energy to initiate the reaction, this is called activation energy.
Enzymes reduce activation energy, allowing reactions to proceed more readily.
An enzyme-catalyzed reaction needs less energy input than if the reaction had no enzyme.

The Activation Energy Barrier

Chemical reactions involve breaking and forming bonds, requiring activation energy.
Activation energy usually arises from thermal energy absorbed from surroundings.
Enzymes catalyze reactions by lowering the activation energy barrier.
Enzymes do not have an effect on the change in free energy (∆G).

Energy Changes during the Reaction

All chemical reactions have an energy barrier between reactants and products, which is referred to as the free energy of activation.
Free energy is a thermodynamic function useful in understanding enzyme-catalyzed reactions.
The change in free energy, denoted as ∆G, is the difference in free energy between reactants and products.
If ∆G is negative, the reaction is spontaneous.
If ∆G is positive, the reaction needs energy input.

Exergonic and Endergonic Reactions

Exergonic reactions release free energy; they are spontaneous.
Endergonic reactions absorb free energy; they are not spontaneous.

ATP powers cellular work by coupling exergonic reactions to endergonic reactions

Cells manage energy resources by energy coupling, using energy from one process to drive another.
ATP is a common energy currency in cells for coupling exergonic and endergonic reactions.

Structure of ATP and its Hydrolysis

ATP consists of an adenine, a ribose sugar, and three phosphate groups connected to each other by high-energy phosphate bonds.
Hydrolysis of ATP releases energy from breaking the terminal phosphate bond.
The energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves.

The Regeneration of ATP

ATP is continuously regenerated from ADP through addition of a phosphate group.
The energy for phosphorylating ADP comes from catabolic reactions within the cell.
The ATP cycle efficiently shuttles energy between catabolism and anabolism.

Mechanisms for Regulating Enzyme Activity

Enzymes' activity regulation is crucial for coordinating metabolic processes.
There are three main mechanisms: allosteric modulation, covalent modification, and induction/repression of enzyme synthesis.

Allosteric Modulation

Allosteric enzymes exist in different structural forms (high vs low).
Allosteric modulators (ligands) bind to a site separate from the active site, affecting the enzyme activity.
Binding may either activate or inhibit the enzyme.
The allosteric site is distinct from the substrate-binding site.

Allosteric Modulators

Allosteric modulators bind to the allosteric site by non-covalent interactions.
These interactions reversibly alter the enzyme's conformation and its activity.

Allosteric Inactivators and Activators

Allosteric activators increase enzyme activity, while inhibitors decrease it.
Activator binding alters conformation to the high-affinity state.
Inhibitor binding promotes low affinity active conformation.

Most Allosteric Enzymes

Most are oligomeric, with multiple subunits (protomers).
Binding of substrate to one subunit may affect binding affinity of other subunits in a positive or negative manner (cooperative).
Some are monomeric enzymes.

Homotropic Effect and Heterotropic Effect

Homotropic effect occurs when the substrate acts as an allosteric modulator, often leading to positive cooperativity.
Heterotopic effect occurs when a different molecule (like an end product) acts as an allosteric modulator, often acting as feedback regulation of the enzyme activity.

Example of Feedback Inhibition

Aspartate transcarbamoylase (ATCase) is an allosteric enzyme crucial for pyrimidine biosynthesis.
CTP is an allosteric inhibitor for ATCase (heterotrophic effect) that effectively regulates the reaction.
ATP, an intermediate in the pathway, acts as an allosteric activator (homotrophic effect) for ATCase.

Covalent Modification

Enzyme activity is regulated by reversible covalent modifications.
Key examples are modification by phosphorylation and dephosphorylation.
Covalent modification alters the charge state of the enzyme's active site influencing its conformation and thus activity.

Example on Reversible Covalent Regulation

Phosphorylation/dephosphorylation by protein kinases and phosphatases typically modify the active site for the enzyme in a reversible manner altering conformation. This results in altered activity.

Irreversible Activation by Proteolytic Cleavage

Some proteins are produced in inactive zymogen forms.
Activation occurs through proteolytic cleavage, a process where a protease cleaves a small portion from the protein.
This triggers a conformational change enabling the zymogen to function in its active form.
Example: activation of trypsin and pepsin from their respective zymogens, trypsinogen, and pepsinogen.

Induction-Repression of Enzyme Synthesis

Enzyme synthesis can be controlled through gene expression.
Enzyme synthesis can increase or decrease in response to cellular conditions (induction/repression).
Changes in the synthesis rate affect intracellular enzyme concentration, hence modifying the enzyme's activity.

Examples of Induction and Repression

Sex hormones, such as estrogen and testosterone, can induce or repress specific enzymes.
Insulin can alter the synthesis rates of enzymes involved in glycolysis and gluconeogenesis.

Isoenzymes

Related enzymes with similar metabolic functions.
Isoenzymes may have different subunit compositions, thus different kinetics and/or regulatory behaviors and/or different tissue distributions.
Different subunit compositions give them distinct properties, like different charge patterns, allowing separation by electrophoresis.

Mechanisms for Production of Isoenzymes

Different combinations of polypeptide subunits create various isoenzyme forms.
Example: creatine kinase (CK) is a dimer with muscle (M) and brain (B) subunits.
Example: lactate dehydrogenase (LDH) is a tetramer with different subunit combinations.

Post-Translational Modification

Post-translational modifications (PTMs) like adding phosphate, acetate, amide or methyl groups can change the properties of enzymes.
This can lead to creation of isoenzymes by covalent addition.

Separation of Isoenzymes

Isoenzyme separation is often achieved through electrophoresis based on the different properties and charges of the different isoenzymes, which are a product of different subunit composition.

Use of Inhibitors

Isoenzyme activity can be inhibited by certain compounds.
This can be helpful for identification.
Example: Alkaline phosphatase (ALP) is inhibited by phenylalanine.

Enzymes in Clinical Diagnosis

Enzyme assays in blood allow for diagnosis of various conditions.
Cellular enzymes are usually localized to specific cells.
During normal turnover, enzymes are released into plasma and then eliminated from the body.
Under pathological conditions, enzyme levels can rise in plasma as a result of cell damage, which can indicate the severity of the pathology, and the specific region damaged.

Transaminases

AST or GOT and ALT or GPT are important in diagnosing diseases of the hepatic, cardiac or skeletal-muscular systems.
Higher levels of transaminases can indicate various conditions including acute pancreatitis.

Alkaline Phosphatases (ALP)

ALP catalyzes the removal of phosphate groups from phosphate esters.
Abundant in liver, bone, intestine and placenta.
Elevated ALP levels can suggest bone or liver pathology

Acid Phosphatase (ACP)

ACP hydrolyzes phosphate from phosphate esters at an acid pH.
It's vital in diagnosing metastatic prostate cancers, since higher levels are associated with the presence and spread of such a cancer.

Lactate Dehydrogenase (LDH)

LDH catalyzes the reversible interconversion of lactate and pyruvate.
Found in most tissues, its elevated levels often indicate conditions like myocardial infarction or hemolytic anemia.
Specific types of LDH, based on different subunit compositions, can be distinguished, thus contributing to the diagnostic value of assessing LDH serum levels.

Creatine Kinase (CK)

CK catalyzes the transfer of phosphate to ADP from creatine phosphate, creating ATP.
Abundant in heart and skeletal muscles, its raised levels often suggest conditions related to muscle damage.
Different isoenzymes occur in different tissues (brain, heart, skeletal muscle) which aids in the diagnostic process, as the elevated levels are tissue specific.

Troponin T and Troponin I

These regulatory proteins are involved in myocardial contractility.
Their release into the plasma is a sensitive marker for cardiac damage, particularly myocardial infarction (MI).

Amylase and Lipase

Amylase and Lipase, digestive enzymes, are often elevated in acute pancreatitis.

Enzymes as Therapeutic Agents

Enzymes are used in disease management via various pathways, including dissolving clots (streptokinase & urokinase), treating cancer (asparaginase), improving digestion (pepsin and trypsin), and managing conditions like emphysema (alpha-1-antitrypsin).

Other Useful Enzymes (in Therapy)

Collagenase breaks down collagen, used in burns.
Lysozyme, an antibacterial enzyme in tears, is used for eye infections.
Penicillinase degrades penicillin and helpful for allergic patients.

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Description

Explore the intricate mechanisms of enzyme regulation, including allosteric activators and covalent modifications. This quiz covers key concepts such as zymogens, enzyme kinetics, and the physiological roles of enzymes in clinical settings. Test your understanding of important enzymes like alkaline phosphatase and aspartate transaminase.