Free ASCP MLS Biochemistry Mock Test: Part 46 – Fundamentals & Enzymes Practice Exam

Red blood cells (RBCs) contain a very high concentration of the LD1 isoenzyme. When a blood specimen is hemolyzed, the RBCs rupture and release their contents, including LD1, into the serum.

This release artificially elevates the measured LD1 fraction on electrophoresis, which can mask the true clinical pattern and lead to a false interpretation, such as incorrectly suspecting a myocardial infarction.

Enzymes act as biological catalysts, speeding up reactions by lowering activation energy.
They are not consumed in the reaction and do not change the equilibrium, only how quickly equilibrium is reached.

Why the other options are false:

• a) Enzymes are consumed during reactions: This is false. Enzymes are catalysts and are regenerated at the end of the reaction cycle. They are not reactants.

• b) Enzymes alter reaction equilibrium: This is false. Enzymes do not change the equilibrium constant (KeqKeq​) of a reaction. They only speed up the rate at which equilibrium is reached. The final equilibrium concentrations of substrates and products remain the same.

• d) Enzymes increase activation energy: This is the opposite of the truth. Enzymes lower the activation energy, which is the energy barrier that must be overcome for a reaction to occur. Lowering this barrier is how they increase the reaction rate.

Allosteric enzymes have regulatory sites where molecules other than the substrate can bind to increase or decrease enzyme activity.
Aspartate transcarbamoylase (ATCase) is a classic example, regulated by feedback inhibition in pyrimidine biosynthesis.

Why the other options are incorrect:

• a) Creatine kinase: While its activity is regulated, this is primarily done through phosphorylation and dephosphorylation, not by the allosteric binding of a small molecule effector. It is not a classic allosteric enzyme.

• c) Amylase & d) Lipase: These are digestive enzymes. Their activity is primarily controlled by substrate availability and hormonal signals (e.g., secretion from the pancreas), not by allosteric regulation via specific effector molecules. They function based on Michaelis-Menten kinetics without complex allosteric control mechanisms.

The “lock and key” model depicts how an enzyme’s active site has a specific shape that perfectly fits its substrate, illustrating the high specificity of enzyme-substrate interactions.

Why the other options are incorrect:

• a) Enzyme denaturation: This refers to the unfolding and loss of function of an enzyme due to factors like heat or pH. The lock and key model describes the functional interaction, not its destruction.

• c) Noncompetitive inhibition: This is a type of inhibition where an inhibitor binds to a site other than the active site. The lock and key model does not account for this allosteric regulation; the “induced fit” model is a better, more dynamic representation that can accommodate inhibition.

• d) Enzyme activation by cofactors: While cofactors are essential for the activity of some enzymes (holoenzymes), the lock and key model specifically describes the geometric complementarity between the enzyme’s active site and the substrate, not the role of additional chemical components.

The Lineweaver-Burk plot is a double reciprocal graph of enzyme kinetics, where 1/V is plotted against 1/[S].

• The y-intercept is equal to 1/Vmax.

• The x-intercept is equal to -1/Km.

Therefore, by taking the negative reciprocal of the x-intercept, you can calculate the Michaelis-Menten constant (Km).

Isoenzymes (or isozymes) are different forms of an enzyme that catalyze the same chemical reaction but are encoded by different genes and thus have different amino acid sequences, and often different kinetic properties or tissue distribution (e.g., Lactate Dehydrogenase LDH-1 and LDH-5).

• a) Apoenzyme: The protein part of an enzyme without its cofactor.

• c) Coenzyme: An organic cofactor.

• d) Allosteric enzyme: An enzyme whose activity is regulated by a molecule binding at a site other than the active site.

Bence Jones proteins are free monoclonal immunoglobulin light chains (kappa or lambda) produced in conditions like multiple myeloma.

• a) Sulfosalicylic Acid Test: This is a nonsensitive test for all proteins; it cannot specifically identify Bence Jones proteins.

• b) Urine Reagent Strip: This primarily detects albumin and is often falsely negative for Bence Jones proteins.

• d) Biuret Reaction: This is a general test for peptide bonds and is not specific.

The Aspartate Aminotransferase (AST) reaction produces oxaloacetate. However, oxaloacetate is not easy to measure directly in a kinetic assay.

Therefore, in a coupled assay, Malate Dehydrogenase (MDH) is added. MDH catalyzes the following reaction:

Oxaloacetate + NADH + H⁺ → Malate + NAD⁺

This coupling reaction serves two purposes:

1. It consumes oxaloacetate, driving the AST reaction forward.

2. It allows the reaction to be monitored by measuring the decrease in absorbance at 340 nm as NADH is converted to NAD⁺.

• Carbonic anhydrase is a crucial enzyme that rapidly interconverts carbon dioxide and water to bicarbonate and protons (and vice versa). The reaction it catalyzes is:
  CO₂ + H₂O ⇌ H⁺ + HCO₃⁻

• This reaction is vital for processes like carbon dioxide transport in the blood, pH regulation, and respiration.

• The active site of carbonic anhydrase contains a zinc ion (Zn²⁺) as an essential cofactor. This zinc ion is coordinated to the imidazole rings of three histidine residues in the enzyme’s structure.

• The zinc ion plays a direct role in the catalytic mechanism by activating a water molecule, making it a much better nucleophile (hydroxide ion) to attack the carbon dioxide molecule.

Why the other options are incorrect:

• a) Iron (Fe): Iron is essential for enzymes like cytochromes, catalase, and peroxidase.

• c) Copper (Cu): Copper is a key cofactor for enzymes like cytochrome c oxidase, superoxide dismutase, and tyrosinase.

• d) Magnesium (Mg): Magnesium is essential for enzymes that utilize ATP (e.g., kinases), as well as DNA polymerase and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).

The biuret reaction is a colorimetric assay where a violet color is produced when peptide bonds (the chemical bonds linking amino acids in a protein) react with copper ions in an alkaline solution. The intensity of the color is proportional to the number of peptide bonds, and thus, the protein concentration.

• a) Free amino groups are detected by other reactions, like the ninhydrin test.

• b) & d) Tyrosine and tryptophan residues are involved in the Lowry method and other specific assays, but not the fundamental biuret reaction.

The sulfhydryl group (-SH) is the functional group present in the amino acid cysteine. This group is highly reactive and plays critical roles in forming disulfide bonds that stabilize protein structure, and in the catalytic site of some enzymes.

• a) Serine has a hydroxyl (-OH) group.

• b) Glycine has only a hydrogen atom as its side chain.

• d) Tyrosine has a phenolic hydroxyl group.

The Regan isoenzyme is a carcinoplacental alkaline phosphatase isoenzyme. This means it is an alkaline phosphatase variant produced by some cancerous tumors (carcino-) that has biochemical and immunologic properties nearly identical to the alkaline phosphatase produced by the placenta.

The Michaelis constant (Km) is defined as the substrate concentration at which the reaction velocity is half of the maximum velocity (Vmax). It is a measure of the enzyme’s affinity for its substrate; a lower Km indicates a higher affinity.

Macroamylasemia is a condition where amylase forms a large complex (macroamylase) with immunoglobulins, most commonly IgA. This complex is too large to be filtered by the kidneys.

• Serum Amylase: The macroamylase gets trapped in the bloodstream, leading to a persistently increased serum amylase level.

• Urine Amylase: Because the complex cannot be filtered, the urine amylase level is normal or low.

At the alkaline pH of 8.6 used in serum protein electrophoresis, proteins are negatively charged and will migrate toward the positive anode (opposites attract). Albumin is the most negatively charged protein in serum at this pH due to its high proportion of acidic amino acids.

Because it carries the strongest negative charge, albumin migrates the fastest and farthest toward the anode, forming the first and most distinct band.

Here’s why the other options are incorrect:

• a) Lipase: This enzyme breaks down lipids (fats) into fatty acids and glycerol.

• b) Lactate Dehydrogenase (LD): This enzyme catalyzes the interconversion of pyruvate and lactate, especially important in anaerobic metabolism.

• d) Trypsin: This is a protease, an enzyme that breaks down proteins into smaller peptides.

Creatinine is a waste product filtered freely by the glomeruli and is not reabsorbed by the renal tubules. Therefore, the rate at which creatinine is cleared from the blood into the urine (creatinine clearance) provides a close and practical estimate of the Glomerular Filtration Rate (GFR), which is the volume of fluid filtered by the glomeruli per unit time.

• The enzyme catalase is found in nearly all living organisms, and its primary biological role is to protect cells from oxidative damage.

• It catalyzes the decomposition of hydrogen peroxide (H₂O₂), a harmful byproduct of metabolic processes, into water and oxygen.

• The chemical reaction is:
  2 H₂O₂ → 2 H₂O + O₂

Let’s see why the other options are incorrect:

• a) Urea: Urea is decomposed by the enzyme urease.

• c) Glucose: Glucose is broken down by a series of enzymes, primarily in glycolysis, but no single enzyme named “catalase” is responsible for this.

• d) Ammonia: Ammonia is a product of various metabolic reactions (like deamination) and is processed by enzymes in the urea cycle, not by catalase.

When substrate concentration is high enough that all enzyme active sites are occupied, the enzyme is saturated, and the reaction rate reaches Vmax.
At this point, increasing substrate further does not increase velocity.

Why the other options are incorrect:

• a) Substrate is very low: At very low substrate concentrations, the velocity is directly dependent on the substrate concentration (the reaction is first-order with respect to substrate).

• b) Enzyme is in excess: If the enzyme is in excess, the reaction velocity will still be directly proportional to the substrate concentration until the point where the substrate becomes limiting.

• d) Reaction temperature is low: A low temperature generally slows down the reaction rate (lowers Vmax) but does not change the fundamental relationship where velocity becomes independent of substrate at saturation.

In the induced-fit model, the enzyme is flexible and adjusts its conformation to better accommodate the substrate.
This differs from the lock-and-key model, where both enzyme and substrate are rigid and fit exactly without conformational changes.

Why the other options are incorrect:

• b) The enzyme and substrate are rigid structures: This is the core principle of the lock-and-key model, not the induced-fit model.

• c) No product is formed: In both models, the ultimate goal is the formation of a product. The difference lies in the mechanism of binding, not the final outcome.

• d) The substrate changes the enzyme permanently: The conformational change in the induced-fit model is temporary and reversible. Once the products are released, the enzyme returns to its original shape, ready to bind another substrate.

Enzyme activity is primarily influenced by factors that directly alter the enzyme’s structure or the kinetics of the reaction it catalyzes.

• a) Temperature affects reaction rate and can denature the enzyme.

• b) pH affects the charge of amino acids, altering the enzyme’s shape and function.

• d) Substrate concentration directly determines the reaction rate according to Michaelis-Menten kinetics.

Normally, the Lactate Dehydrogenase (LD) isoenzyme pattern in serum shows LD2 > LD1.

A “flipped” pattern (where LD1 > LD2) is highly characteristic of myocardial infarction (heart attack). This occurs because the heart muscle (myocardium) is rich in the LD1 isoenzyme. When heart muscle cells are damaged, they release LD1 into the bloodstream, causing its level to exceed that of LD2.

• a) Viral Hepatitis primarily affects liver enzymes (ALT, AST).

• c) Pancreatitis is associated with elevated amylase and lipase.

• d) Renal Failure does not cause a specific flip in the LD isoenzyme pattern.

Denaturation disrupts an enzyme’s secondary, tertiary, or quaternary structure without breaking peptide bonds, causing the active site to lose its shape and the enzyme to lose its catalytic activity.

Why the other options are incorrect:

• a) Increased catalytic efficiency: Denaturation destroys the enzyme’s function, it does not improve it. Catalytic efficiency is optimized at the enzyme’s natural, folded state.

• c) Permanent activation: Denaturation leads to permanent inactivation, not activation.

• d) No change in active site conformation: This is the opposite of what happens. The change in the active site’s conformation is the direct cause of the activity loss during denaturation.

In an enzyme reaction progress curve, the initial phase is linear (zero-order kinetics) because the substrate concentration is high and constant. A deviation from this linearity shortly after the reaction starts most commonly occurs because the substrate concentration has decreased significantly, and the reaction is shifting from zero-order to first-order kinetics with respect to substrate.

• a) The reaction reaching equilibrium: This would cause the curve to plateau, but it typically happens later, not shortly after initiation.

• c) Enzyme denaturation: This usually occurs over a much longer time frame.

• d) The presence of an inhibitor: This would typically affect the initial linear rate (slope), not cause an early deviation from linearity.

A coenzyme is a specific type of cofactor. It is a small, organic molecule (often a vitamin derivative) that is loosely bound to an enzyme and is essential for its catalytic activity. Coenzymes often act as carriers of functional groups or electrons during the reaction.

• a) Describes a prosthetic group, which is a tightly or permanently bound cofactor.

• c) An enzyme inhibitor blocks activity.

• d) A metallic activator is an inorganic cofactor (e.g., Mg²⁺, Zn²⁺).

Vitamin C (ascorbic acid) acts as a coenzyme in certain oxidation-reduction reactions, donating electrons to assist in enzymatic reactions, such as collagen synthesis and hydroxylation reactions.

Why the other options are less central as coenzymes in redox reactions:

• b) Vitamin D: Its primary role is as a hormone in calcium and phosphate metabolism, not as a coenzyme in redox reactions.

• c) Vitamin A: While its derivative, retinal, is involved in the visual cycle (a process involving isomerization, not classic electron transfer), it is not a primary coenzyme for general metabolic redox reactions like Vitamin C or B vitamins.

• d) Vitamin K: It acts as a coenzyme in the carboxylation of glutamate residues in clotting factors and other proteins, which is not an oxidation-reduction reaction.

A peptide bond is the specific, covalent chemical bond that forms between the carboxyl group (-COOH) of one amino acid and the amino group (-NH₂) of another, releasing a water molecule. This bond is the primary linkage that creates the linear backbone of a protein chain (polypeptide).

• a) Hydrogen bonds are important for the secondary and tertiary structure of proteins.

• c) Ionic bonds can form between charged side chains in the tertiary structure.

• d) Disulfide bonds are strong covalent bonds between cysteine residues that stabilize the tertiary/quaternary structure.

Isoenzymes (isozymes) are different forms of the same enzyme that catalyze the same chemical reaction but have different amino acid sequences, structures, or tissue distributions.

Why the other options are incorrect:

• a) It has identical amino acid sequences but different cofactors: If the amino acid sequences were identical, it would be the same enzyme. Different cofactors would not make it an isoenzyme; it would just be the same protein under different conditions.

• b) It catalyzes different reactions: By definition, enzymes that catalyze different reactions are different enzymes, not isoenzymes.

• d) It is inactive at all temperatures: This describes an inactive protein or denatured enzyme, not an isoenzyme. Isoenzymes are functionally active.

Gamma-Glutamyl Transferase (GGT) is the most sensitive enzymatic indicator for liver damage from chronic alcohol intake. While not entirely specific for alcohol, it is highly inducible by ethanol and other drugs. In chronic alcohol consumption, GGT levels are often disproportionately elevated compared to other liver enzymes like ALT and AST.

• a) ALT is more specific for general liver cell damage (e.g., viral hepatitis).

• b) AST is found in the liver, heart, and muscle, and is less specific.

• c) ALP is more indicative of biliary tract obstruction.

1 International Unit (IU) of enzyme activity is the amount of enzyme that catalyzes the conversion of 1 micromole (μmol) of substrate per minute under standard conditions (usually at 25°C, optimal pH).

The primary function of serum albumin in the blood is to maintain colloidal osmotic pressure (also known as oncotic pressure). This pressure is essential for regulating the distribution of fluid between the blood vessels and the tissues, preventing edema and ensuring proper blood volume.

While serum albumin also plays a role in transporting substances such as fatty acids, bilirubin, and drugs, this is not its primary function. It does not act as an antibody (antibodies are immunoglobulins), nor does it serve as a cofactor for enzymatic reactions.

A competitive inhibitor is a molecule that closely resembles the substrate in its molecular structure. It binds reversibly to the enzyme’s active site, competing directly with the substrate for binding. This inhibition can be overcome by increasing the substrate concentration.

The active site is the specific region on the enzyme where the substrate binds and the chemical reaction is catalyzed. It is typically a pocket or cleft with a unique three-dimensional shape and chemical environment that is complementary to the substrate.

• a) Regulatory site / d) Allosteric center: This is a different site where regulatory molecules (allosteric effectors) bind to influence the enzyme’s activity.

• c) Secondary structure: This refers to local folding patterns (like alpha-helices) that are part of the enzyme’s overall structure, not the binding site itself.

In competitive inhibition, the inhibitor binds to the enzyme’s active site, competing directly with the substrate.
This means:

• KmKm​ (Michaelis constant) increases because a higher substrate concentration is needed to reach half of VmaxVmax​ due to the competition.

• VmaxVmax​ remains unchanged because, at high substrate concentrations, the substrate can outcompete the inhibitor and the same maximum rate is achievable.

Enzyme activity is determined by measuring the rate at which substrate is converted into product (or substrate disappears) over time.
This reflects the catalytic efficiency of the enzyme under specified conditions.

Why the other options are incorrect:

• a) Product disappearance: This describes the reverse reaction or the breakdown of a product, not the primary activity of the enzyme forming the product.

• b) Enzyme color: While some enzymes are colored, their color does not change with activity in a way that is useful for general activity measurement. Some assays use colorimetric changes in the substrate or product to track the reaction, but not the enzyme’s intrinsic color.

• d) Change in enzyme mass: The mass of the enzyme itself remains constant during catalysis. It is not consumed or changed in mass; it is a catalyst that facilitates the conversion of substrate to product.

In humans, the final product of purine (adenine and guanine) catabolism is uric acid. Most other mammals possess the enzyme uricase, which further breaks down uric acid into the more soluble allantoin.

• a) Urea is the end product of protein/amino acid metabolism.

• c) Allantoin is the end product in many other mammals, but not in humans.

• d) Ammonia is a toxic intermediate in both purine and urea metabolism.

Thymolphthalein monophosphate is considered the most specific substrate for quantifying prostatic acid phosphatase because the prostatic isoenzyme has a much higher affinity for it compared to acid phosphatase from other tissues (like red blood cells or platelets).

Using this substrate minimizes interference from non-prostatic acid phosphatases, leading to a more accurate measurement of the prostatic fraction.

The bone-derived isoenzyme of alkaline phosphatase is the most heat-labile, meaning it is the most easily inactivated by heat. This property is used in clinical laboratories to distinguish it from other isoenzymes, particularly the liver isoenzyme, which is more heat-stable.

For example, heating a serum sample to 56°C for 10 minutes will typically inactivate most of the bone ALP while a significant portion of the liver ALP remains active.

The most critical first step is to measure the total volume accurately. All quantitative results for a 24-hour urine collection (e.g., total protein, creatinine clearance) are expressed as an amount per 24 hours. This calculation requires the total volume to be known first.

• a) Preservatives should be added at the start of the collection, not as a first step in analysis.

• b) A dipstick is a qualitative/semi-quantitative screen and is not a required first step for a quantitative analysis.

• d) Subculturing is not part of a standard protein quantification protocol.

Amylase catalyzes the breakdown of starch (a polysaccharide) into maltose (a disaccharide) by hydrolyzing the α-1,4-glycosidic bonds.

Why the other options are incorrect:

• b) Lipase: This enzyme breaks down lipids (fats and oils) into fatty acids and glycerol. It does not act on carbohydrates like starch.

• c) Protease: This enzyme breaks down proteins into peptides and amino acids. It does not act on carbohydrates.

• d) Lactase: This is a specific enzyme that breaks down the disaccharide lactose (milk sugar) into glucose and galactose. It does not act on starch.

Lactate dehydrogenase (LDH) catalyzes the reversible conversion of lactate ↔ pyruvate and requires NAD⁺ as a coenzyme to accept or donate electrons during the oxidation-reduction reaction.

Why the other options are incorrect:

• b) ATP: ATP is an energy currency molecule, not a coenzyme for electron transfer in the LDH reaction. Enzymes like kinases use ATP.

• c) FADH₂: This is a coenzyme for other dehydrogenases, such as succinate dehydrogenase in the Krebs cycle, but not for lactate dehydrogenase.

• d) CoA (Coenzyme A): CoA is involved in activating acyl groups (e.g., in the formation of acetyl-CoA), not in the lactate-pyruvate conversion.

Ligases are enzymes that join two molecules together, often requiring ATP for energy.
Example: DNA ligase, which joins DNA fragments during replication or repair.

Why the other options are incorrect:

• a) Hydrolase: This class of enzymes catalyzes the cleavage of bonds by the addition of water (hydrolysis). They break molecules apart, and while they may use water, they do not typically use ATP to join molecules together. (e.g., proteases, nucleases, lipases).

• c) Transferase: This class catalyzes the transfer of a functional group (e.g., a methyl, phosphate, or amino group) from one molecule to another. They do not typically join two large molecules using ATP energy (e.g., transaminases, kinases).

• d) Lyase: This class catalyzes the cleavage (or addition) of bonds by means other than hydrolysis or oxidation, often forming a double bond or adding a group to a double bond. They break or form bonds without using ATP (e.g., decarboxylases, dehydratases).

An apoenzyme is the protein portion of an enzyme that is inactive until it binds with its cofactor (which may be a metal ion or an organic molecule).

When the apoenzyme combines with its cofactor, it forms the active enzyme, known as the holoenzyme.

As the name implies, oxidoreductases are the class of enzymes that catalyze oxidation-reduction reactions. These reactions involve the transfer of electrons from one molecule (the reductant) to another (the oxidant).

• a) Transferases transfer functional groups (e.g., aminotransferases).

• c) Hydrolases catalyze hydrolysis reactions (breaking bonds with water).

• d) Isomerases catalyze intramolecular rearrangements.

C-reactive protein (CRP) is a prototypical acute-phase reactant. Its plasma concentration rises dramatically (by hundreds or thousands of times) in response to inflammation, infection, or tissue injury. It is synthesized by the liver and helps in the body’s innate immune response.

Transferases are the class of enzymes that catalyze the transfer of a functional group (such as a phosphate, methyl, or amino group) from one molecule (the donor) to another (the acceptor). Kinases are a common type of transferase that specifically transfer phosphate groups.

• a) Ligases join two molecules using energy from ATP.

• c) Hydrolases break bonds using water.

• d) Lyases remove groups to form double bonds or add groups to double bonds.

While an enzyme like Creatine Kinase (CK) or Lactate Dehydrogenase (LD) can be elevated from damage to several different tissues, its isoenzymes are tissue-specific.

• Measuring the total enzyme activity provides sensitivity (it detects that damage has occurred).

• Measuring the specific isoenzyme (e.g., CK-MB for heart muscle, LD1 for heart) provides specificity by pinpointing the exact tissue of origin.

Amino acids are the monomers or building blocks of proteins. They link together via peptide bonds to form polypeptide chains, which then fold into functional protein structures.

• a) Monosaccharide: The building block of carbohydrates.

• b) Fatty acid: A major component of lipids.

• d) Nucleotide: The building block of nucleic acids (DNA/RNA).

The Henderson-Hasselbalch equation is used to calculate the pH of a buffer solution. It is derived from the acid dissociation constant (Ka) expression.

For a weak acid (HA) and its conjugate base (A⁻), the equation is:

pH = pKa + log( [A⁻] / [HA] )

Where:

• [A⁻] is the concentration of the conjugate base (often the “salt”).

• [HA] is the concentration of the weak acid.

The bicarbonate–carbonic acid (HCO₃⁻/H₂CO₃) system is the main extracellular buffer in human blood.
It maintains blood pH around 7.4 by neutralizing acids and bases through the reversible reaction:

CO2+H2O↔H2CO3↔H++HCO3−CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-CO2​+H2​O↔H2​CO3​↔H++HCO3−​

Why the other options are incorrect:

• a) Phosphate buffer (HPO₄²⁻/H₂PO₄⁻): This is an important buffer system inside cells and in urine, but its concentration in the blood is too low to play a primary role in blood buffering.

• b) Protein buffer: Proteins, especially hemoglobin in red blood cells and plasma proteins, do contribute significantly to the blood’s buffering capacity (they are the second most important buffer). However, they are not considered the primary system. Hemoglobin is crucial for buffering the H⁺ generated from the carbonic acid formed when CO₂ is transported.

• d) Ammonium buffer: This system is primarily used by the kidneys to excrete acid in the urine, but it is not a significant direct buffer in the blood plasma itself.

Oxidoreductases are enzymes that catalyze oxidation-reduction reactions. The key identifier is that they often have “dehydrogenase” in their name, as they typically catalyze reactions involving the removal or addition of hydrogen atoms (i.e., oxidation or reduction). All the enzymes listed in option b are dehydrogenases and are therefore classified as oxidoreductases.

• a) These are all hydrolases (they break down molecules using water).

• c) Aminotransferases are transferases, and creatine kinase is also a transferase.

• d) These are all hydrolases.

The troponin complex is a regulatory protein complex located on the thin filament of striated muscle (cardiac and skeletal). It is composed of three distinct subunits:

• Troponin C (TnC): Binds calcium ions.

• Troponin I (TnI): Inhibits the actin-myosin interaction.

• Troponin T (TnT): Binds the complex to tropomyosin.

The standard assay for Lactate Dehydrogenase (LD) measures the forward (lactate to pyruvate) or, more commonly, the reverse (pyruvate to lactate) reaction.

In the common reverse reaction:
Pyruvate + NADH + H⁺ → Lactate + NAD⁺

The reaction consumes NADH, which absorbs light strongly at 340 nm. The rate of decrease in absorbance at 340 nm is directly measured and is proportional to the LD enzyme activity. Therefore, NADH is the molecule whose concentration is directly monitored.

While the primary structure (the amino acid sequence) is held together by peptide bonds, the three-dimensional structure (secondary, tertiary, and quaternary) is primarily stabilized by weaker, non-covalent interactions:

• Hydrogen bonds: Form between polar side chains and the protein backbone.

• Ionic interactions: Occur between positively and negatively charged amino acid side chains.

• Hydrophobic forces: Cause nonpolar side chains to cluster together in the protein’s interior, away from water.

A holoenzyme is the complete, active form of an enzyme made up of the apoenzyme (protein part) and its cofactor (which may be a metal ion or coenzyme).
Apoenzyme + Cofactor → Holoenzyme (active enzyme)

Why the other options are incorrect:

• a) Apoenzyme only: This is the inactive protein portion. Without its cofactor, it cannot catalyze the reaction.

• b) Coenzyme only: A coenzyme is just one type of cofactor. It cannot function as an enzyme on its own without the apoenzyme.

• d) Substrate and product: These are the molecules the enzyme acts upon (substrate) and the molecules it produces (product). They are not part of the enzyme’s structure.

Enzymes are biological catalysts that work by lowering the activation energy of a reaction. The activation energy is the energy barrier that must be overcome for a reaction to proceed. By lowering this barrier, enzymes allow the reaction to occur much faster and at a lower temperature, without being consumed or altering the reaction’s equilibrium.

The classic Jaffe reaction for creatinine quantification involves the reaction of creatinine with picric acid in an alkaline medium (sodium hydroxide) to form an orange-red complex. The intensity of the color is proportional to the creatinine concentration.

Creatine Kinase-MB (CK-MB) is an isoenzyme of creatine kinase that is found predominantly in the heart muscle (myocardium). While other tissues contain different forms (isoenzymes) of CK, a markedly elevated level of CK-MB in the serum is a highly specific and classic biomarker for damage to the heart muscle, such as in a myocardial infarction (heart attack).

• a) Liver damage is more associated with elevated enzymes like ALT and AST.

• b) Skeletal muscle contains mainly the CK-MM isoenzyme.

• d) Brain tissue contains the CK-BB isoenzyme.