Free ASCP MLS Biochemistry Mock Test: Part 48 – Proteins, Amino Acids & Plasma Proteins Practice Exam

• Transferrin is a glycoprotein in plasma that binds free iron (Fe³⁺) and transports it to tissues, especially the bone marrow for hemoglobin synthesis.

• It helps keep iron soluble and non-toxic in the bloodstream.

Other options:

• a) Transport copper → Ceruloplasmin does this.

• c) Carry oxygen → Hemoglobin carries oxygen.

• d) Bind bilirubin → Albumin binds bilirubin in plasma.

C-reactive protein (CRP) is a major positive acute-phase reactant, meaning its plasma concentration increases during inflammation, infection, or tissue injury.

The other options are negative acute-phase reactants (their levels decrease during inflammation):

• a) Albumin

• b) Transferrin

• d) Prealbumin

The recommended method for measuring total protein in serum is based on which reaction?

The biuret method is the recommended routine clinical chemistry procedure for measuring total protein in serum.

• It is based on the reaction of peptide bonds with copper ions in an alkaline solution to form a violet-colored complex.

• The intensity of the color is proportional to the number of peptide bonds and thus the protein concentration.

Other options:

• (a) Molybdenum blue → Used in phosphorus determination.

• (b) Ferri-ferrocyanide → Used in the glucose (Folin–Wu) method.

• (c) Resorcinol-HCl → Used for fructose estimation.

The biuret reaction is a chemical test used to detect the presence of proteins. It depends on the presence of two or more peptide bonds in the molecule.

When protein (or any compound containing peptide bonds) reacts with copper(II) sulfate in an alkaline medium, a violet or purple-colored complex forms. The intensity of this color is directly proportional to the number of peptide bonds, and thus to the protein concentration.

Other options:

• (a) Free amino groups → Not the main factor in the biuret reaction.

• (b) Free carboxyl groups → Not involved in the color reaction.

• (d) Tyrosine residues → Measured in other reactions (e.g., Lowry or Folin–Ciocalteu), not in the biuret test.

Ceruloplasmin is a copper-binding α₂-globulin synthesized in the liver. It binds about 90–95% of the total serum copper, serving both as a transport protein and as an enzyme (ferroxidase) that helps oxidize Fe²⁺ → Fe³⁺ for iron transport by transferrin.

On serum protein electrophoresis (SPEP), ceruloplasmin migrates in the α₂-globulin region — along with haptoglobin and α₂-macroglobulin.

Other options:

• (a) Albumin region → Contains mainly albumin and some prealbumin.

• (b) Alpha-1 region → Contains α₁-antitrypsin and α₁-acid glycoprotein.

• (d) Beta region → Contains transferrin, β-lipoprotein, and complement C3.

Plasma albumin has two major functions:

1. Maintain colloidal osmotic pressure (oncotic pressure), which helps regulate fluid distribution between blood vessels and tissues.

2. Transport various substances, such as hormones, fatty acids, bilirubin, and drugs.

Other options (incorrect):

• (a) Act as an enzyme → Albumin is not an enzyme.

• (c) Serve as an antibody → That’s the role of immunoglobulins (gamma globulins).

• (d) Facilitate blood coagulation → That’s the role of fibrinogen and clotting factors.

In humans, uric acid is the primary end product of purine metabolism (breakdown of adenine and guanine nucleotides).

• Most other mammals further break down uric acid to allantoin via the enzyme uricase, but humans lack this enzyme, so uric acid is the final product.

Other options:

• (a) AMP and GMP → These are purine nucleotides, not breakdown products.

• (b) DNA and RNA → Contain purines but are not metabolic end products.

• (c) Allantoin → Formed from uric acid in most other mammals, but not in humans (humans lack uricase).

Tryptophan (along with tyrosine and phenylalanine) absorbs ultraviolet light at 280 nm due to its aromatic ring structure. This property is used to measure protein concentration spectrophotometrically.

Other options:

• a) Glycine → Non-aromatic, does not absorb at 280 nm.

• c) Alanine → Non-aromatic, no significant absorption.

• d) Serine → Non-aromatic, no significant absorption.

Bence Jones proteins (free immunoglobulin light chains) have a unique thermal solubility property:

• They precipitate when urine is heated to 40–60°C.

• They re-dissolve when the temperature is increased to near 100°C.

Other options:

• (a) Ammonium sulfate → Used for protein precipitation in general, not specific for Bence Jones protein.

• (b) Sulfuric acid → Not used for detecting proteins.

• (d) At 100 °C → Bence Jones proteins redissolve, not precipitate, at this temperature.

In nephrotic syndrome:

• Serum albumin is decreased due to massive urinary loss of albumin.

• Alpha-2 globulins are increased because larger proteins like alpha-2-macroglobulin are retained and produced in higher amounts by the liver to compensate.

Other options:

• b) Cirrhosis → ↓ albumin, ↑ γ-globulins (“β–γ bridging”).

• c) Multiple myeloma → Sharp ↑ in γ-globulins (monoclonal spike).

• d) Inflammation → ↑ α₁ and α₂ globulins, but albumin decreases moderately.

Oligoclonal bands in the cerebrospinal fluid (CSF) represent the presence of restricted immunoglobulin populations, indicating intrathecal antibody production.

This finding is a key laboratory feature used to support the diagnosis of multiple sclerosis, an autoimmune demyelinating disease of the central nervous system.

Other options:

• (a) Multiple myeloma → Detected by serum/urine electrophoresis for monoclonal (M) protein, not CSF OCBs.

• (c) Myasthenia gravis → Diagnosed via acetylcholine receptor antibody tests, not CSF analysis.

• (d) von Willebrand disease → A bleeding disorder, diagnosed with factor and antigen assays, not CSF studies.

In serum protein electrophoresis (SPEP), proteins are separated based on their size, shape, and electric charge when an electric current is applied to a buffered medium (usually at pH 8.6 on agarose gel or cellulose acetate).

At this alkaline pH, proteins are negatively charged and migrate toward the anode (positive electrode) — the more negative the charge, the faster the migration.

• Pepsin is an enzyme secreted by the chief cells of the stomach in an inactive form called pepsinogen. It is activated by the acidic environment (HCl) of the stomach and is responsible for digestion of proteins into smaller peptides.

Other options:

• b) Trypsin → Protein digestion, but occurs in the small intestine, secreted by the pancreas as trypsinogen.

• c) Amylase → Digests carbohydrates, not proteins.

• d) Lipase → Digests fats, not proteins.

Bromcresol green (BCG) and bromcresol purple (BCP) are dyes used in clinical chemistry to measure serum albumin. They bind preferentially and with the highest capacity to albumin compared to other serum proteins. This property makes them useful for specific albumin quantification.

Other options:

• (a) Alpha-1 globulin, (c) Beta globulin, and (d) Gamma globulin do not bind these dyes as strongly or specifically as albumin.

Amino acids are the fundamental structural units (monomers) that link together via peptide bonds to form proteins (polypeptides).

Other options (incorrect):

• (a) Monosaccharides → Building blocks of carbohydrates

• (c) Fatty acids → Building blocks of lipids

• (d) Nucleotides → Building blocks of nucleic acids (DNA and RNA)

When comparing plasma to serum protein electrophoresis:

• Plasma contains fibrinogen, which migrates in the beta-gamma region (often between the beta and gamma globulins) and appears as a sharp, distinct peak.

• Serum does not contain fibrinogen (it is converted to fibrin during clotting), so this peak is absent.

Other options:

• (a) Broad prealbumin peak → Not related to plasma vs serum difference.

• (c) Diffuse pattern due to anticoagulants → Anticoagulants do not cause a diffuse pattern.

• (d) Decreased globulin fraction → Plasma and serum have similar globulin levels.

The secondary structure of a protein (such as alpha-helices and beta-pleated sheets) is stabilized primarily by hydrogen bonds between the backbone carbonyl oxygen (C=O) and amino hydrogen (N-H) groups of peptide bonds.

The correct answer is:
✅ b) Hydrogen bonds

Explanation:

The secondary structure of a protein refers to the local folding patterns of the polypeptide backbone, primarily into α-helices and β-pleated sheets.

These structures are stabilized mainly by hydrogen bonds between:

• The carbonyl oxygen (C=O) of one peptide bond, and

• The amide hydrogen (N–H) of another peptide bond within the same or nearby chain.

Types of secondary structure:

1. α-Helix:

   • Right-handed coil.

   • Stabilized by intrachain hydrogen bonds (every 4th amino acid).

   • Example: Myoglobin, hemoglobin.

2. β-Pleated Sheet:

   • Polypeptide chains lie side-by-side.

   • Stabilized by interchain or intrachain hydrogen bonds.

   • Example: Fibroin (silk).

Other options (incorrect):

• (a) Ionic bonds: Important in tertiary and quaternary structure (between charged side chains).

• (c) Peptide bonds: Form the primary structure (link amino acids together).

The urine albumin-to-creatinine ratio (UACR) is the preferred and most practical method for detecting and monitoring microalbuminuria/albuminuria.

• It can be performed on a random urine specimen.

• It corrects for variations in urine concentration.

• It is recommended by clinical guidelines for screening diabetic patients and others at risk for kidney disease.

Other options:

• (a) Fructosamine: Reflects short-term blood glucose control, not kidney function.

• (c) Pre-albumin: A plasma protein used to assess nutritional status, not urinary albumin.

• (d) Beta-2 microglobulin: Marker for tubular function, not glomerular (albumin) leakage.

In biliary cirrhosis (primary biliary cholangitis), there is often a prominent elevation in IgM and other immunoglobulins due to chronic immune-mediated damage to the bile ducts. This results in a polyclonal increase in the gamma globulin fraction on serum protein electrophoresis.

Other options:

• (a) Alpha-1 → Normal or slightly decreased

• (b) Alpha-2 → Usually normal

• (c) Beta → May be slightly increased but not characteristic

• Prealbumin (transthyretin) is a plasma protein with a short half-life (~2 days), making it a sensitive marker for recent changes in protein intake and nutritional status.

• Low levels indicate protein-energy malnutrition or inadequate dietary protein.

Other options:

• a) Chronic liver disease → Albumin is more commonly used.

• c) Kidney function → Creatinine and urea are standard markers.

• d) Hemolytic anemia → Haptoglobin and bilirubin are relevant markers.

Nephrotic syndrome causes a decrease in total serum protein due to massive loss of proteins (especially albumin) in the urine.

The other options are more likely to cause normal or increased total protein:

• a) Dehydration → hemoconcentration increases total protein.

• c) Chronic infection → may increase acute-phase proteins.

• d) Multiple myeloma → produces M-protein, often increasing total protein.

• Methionine is an essential amino acid that contains sulfur in its side chain (–CH₂–CH₂–S–CH₃).

• Another sulfur-containing amino acid is cysteine (–CH₂–SH).

Other options:

• a) Valine → Nonpolar, no sulfur.

• c) Alanine → Nonpolar, no sulfur.

• d) Glycine → Simplest amino acid, no sulfur.

The normal albumin to globulin (A/G) ratio in serum is typically in the range of 1.2:1 to 2.0:1.

• Albumin normally makes up a larger proportion of total serum protein than globulins.

• A decreased A/G ratio can occur in conditions such as chronic inflammation, liver disease, or monoclonal gammopathies.

Other options:

• a) 0.5:1 → Low A/G ratio, seen in chronic infections or multiple myeloma.

• b) 1.0:1 → Slightly low-normal.

• d) 3.5:1 → Abnormally high, rarely seen, could indicate hypogammaglobulinemia.

Serum albumin is the most abundant plasma protein in the blood. Its main function is to maintain the colloidal (oncotic) osmotic pressure, which helps keep fluid within the blood vessels and prevents it from leaking into tissues (thus preventing edema).

Other options:

• (a) Antibody production → Done by plasma cells, not albumin.

• (c) Fibrinogen formation → Fibrinogen is a different plasma protein made by the liver for blood clotting.

• (d) Blood viscosity → Determined mostly by red blood cells, not albumin.

In nephrotic syndrome, large amounts of low-molecular-weight proteins (especially albumin) are lost through the damaged glomeruli into the urine.
As a result:

• Serum albumin → Decreases (hypoalbuminemia)

• α₂-globulins (especially α₂-macroglobulin) → Increase (they are large molecules and retained in plasma)

• β-globulins (especially β-lipoprotein) → Increase (due to altered lipid metabolism)

• γ-globulins → May be normal or slightly decreased

Thus, the electrophoretic pattern in nephrotic syndrome shows:

• ↓ Albumin

• ↑ α₂-globulin

• ↑ β-globulin

Other options:

• (a) α₁-globulin → Usually normal or slightly decreased.

• (b) Albumin → Decreased, not elevated.

• (d) β and γ globulins → γ often decreased, not increased.

A peptide bond is a covalent chemical bond formed between the amino group (–NH₂) of one amino acid and the carboxyl group (–COOH) of another amino acid, releasing a molecule of water (H₂O) in a condensation reaction.

Other options (incorrect):

• (a) Two amino groups → Cannot form peptide bonds.

• (b) Two carboxyl groups → No peptide linkage possible.

• (d) Two hydroxyl groups → Involved in ester formation, not peptide bonds.

Maple syrup urine disease is caused by a defect in the branched-chain α-ketoacid dehydrogenase complex, which leads to the accumulation of the branched-chain amino acids valine, leucine, and isoleucine (and their corresponding ketoacids) in the blood and urine.

Other options:

• (a) Phenylalanine → Elevated in phenylketonuria (PKU)

• (b) Tyrosine → Elevated in tyrosinemia

• (d) Cystine and cysteine → Elevated in cystinuria

The bromocresol green (BCG) method is commonly used to measure serum albumin. BCG binds specifically to albumin, producing a color change that is proportional to the albumin concentration.

Other options (incorrect):

• (a) Total protein → Measured by the Biuret method, not BCG.

• (c) Globulins → Calculated as Total protein – Albumin; not measured directly by BCG.

• (d) Fibrinogen → Measured by clot-based assays, not by dye-binding methods.

Tryptophan is the precursor for serotonin (5-hydroxytryptamine), a neurotransmitter involved in mood, sleep, and appetite regulation.

Other options:

• a) Phenylalanine → Precursor for tyrosine, dopamine, norepinephrine, and epinephrine.

• c) Tyrosine → Precursor for dopamine, norepinephrine, epinephrine, and thyroid hormones.

• d) Glycine → Inhibitory neurotransmitter, not a serotonin precursor.

A monoclonal spike (M-protein) on serum protein electrophoresis is a classic finding in multiple myeloma, a malignancy of plasma cells that produce a single, identical immunoglobulin or immunoglobulin fragment.

The troponin complex is a regulatory protein complex located on the thin (actin) filaments of striated muscle (skeletal and cardiac).
It plays a key role in muscle contraction by regulating the interaction between actin and myosin in response to calcium.

Other options (incorrect):

• (a) Troponin T, calcium, and tropomyosin → Missing Troponin I and Troponin C.

• (c) Troponin I, actin, and tropomyosin → Actin is not part of the troponin complex.

• (d) Troponin C, myoglobin, and actin → Myoglobin is a separate oxygen-binding protein, not part of the complex.

The α₁ peak on serum protein electrophoresis is composed primarily of alpha-1 antitrypsin. A large decrease in this peak is a classic finding in alpha-1 antitrypsin deficiency.

• a) An acute phase reaction → would typically increase the α₁ peak.

• c) Chronic inflammation → can alter various fractions but is not the most indicative of a drastically reduced α₁ peak.

• d) Nephrotic syndrome → causes loss of proteins, but affects albumin most prominently and can also reduce α₁, though the most specific answer here is alpha-1 antitrypsin deficiency.

Haptoglobin is an acute-phase protein that binds free hemoglobin released from red blood cells.

• In hemolytic anemia, there is increased intravascular hemolysis, leading to high free hemoglobin levels. This causes haptoglobin to be consumed as it forms complexes with hemoglobin, which are rapidly cleared by the liver, resulting in low serum haptoglobin.

The other options are incorrect:

• a) Decreased in tissue injury/neoplasia → No, haptoglobin is an acute-phase reactant and typically increases in inflammation, tissue injury, and malignancy.

• b) Increased with prosthetic heart valves → No, mechanical heart valves cause chronic hemolysis, which decreases haptoglobin.

• d) Increased in in vitro hemolysis → No, in vitro hemolysis also releases hemoglobin, which can bind and lower measurable haptoglobin.

C-reactive protein (CRP) is an acute-phase protein produced by the liver in response to inflammation, infection, or tissue injury. Its presence is a nonspecific but sensitive marker of an active inflammatory process.

Other options:

• (a) Recent streptococcal infection → Better indicated by antistreptolysin O (ASO) titers, not CRP.

• (b) Recovery from pneumococcal infection → CRP levels decrease during recovery, not increase.

• (d) State of hypersensitivity → Involves immune mechanisms, not specifically reflected by CRP.

A “beta-gamma bridge” (or beta-gamma fusion) is a characteristic finding on serum protein electrophoresis in which there is no clear separation between the beta and gamma globulin regions.

Other options:

• (a) Acute inflammation → Mild increases in α₁ and α₂ globulins.

• (c) Nephrotic syndrome → ↑ α₂ and β peaks, ↓ albumin and γ.

• (d) Monoclonal gammopathy → Shows a sharp spike (M-protein) in the γ region, not a bridge.

A tall, narrow spike (often called an M-spike or M-component) in the gamma region on serum protein electrophoresis is characteristic of a monoclonal gammopathy.

This pattern indicates the presence of a large amount of a single, identical immunoglobulin (or part of an immunoglobulin, such as light chains) produced by a clone of plasma cells, as seen in conditions like:

• Multiple myeloma

• Waldenström’s macroglobulinemia

• Monoclonal gammopathy of undetermined significance (MGUS)

The other options are incorrect because:

• a) Cirrhosis → typically shows a broad, polyclonal increase in gamma globulins.

• b) Acute inflammation → generally shows an increase in acute-phase reactants (alpha-1 and alpha-2 globulins), not a narrow gamma spike.

• d) Polyclonal gammopathy → produces a broad-based increase in the gamma region, not a narrow spike.

The C3 component of complement migrates in the beta globulin region on serum protein electrophoresis using cellulose acetate at pH 8.6.

Migration summary on SPEP (pH 8.6):

• Albumin region → Albumin

• α₁ region → α₁-antitrypsin, α₁-acid glycoprotein

• α₂ region → Haptoglobin, ceruloplasmin, α₂-macroglobulin

• β region → Transferrin, complement C3, β-lipoprotein

• γ region → Immunoglobulins (IgG, IgA, IgM)

The quaternary structure of a protein refers to the arrangement and interaction of two or more polypeptide chains (subunits) to form a functional protein complex.

Other options (incorrect):

• (a) A single polypeptide chain → Describes primary, secondary, or tertiary, not quaternary.

• (b) Alpha-helix → A secondary structure element.

• (d) Beta-turn → Also a secondary structure motif.

An albumin excretion rate of 50 µg/min falls within the range for microalbuminuria (typically defined as 20–200 µg/min or 30–300 mg/24 hours).

• This level is above normal (<20 µg/min) but below the threshold for overt proteinuria (>200 µg/min).

• It is an early sign of kidney damage, especially in diabetic nephropathy.

Other options:

• (a) Hematuria → Presence of red blood cells in urine.

• (b) Massive proteinuria → >3.5 g/day (seen in nephrotic syndrome).

• (d) Oliguria → Low urine output (<400 mL/day).

Serum albumin concentration can appear increased in dehydration due to loss of plasma water, which causes hemoconcentration (the proteins are more concentrated, but total body albumin is not actually increased).

Other options:

• (a) Nephrotic syndrome → Albumin decreases due to loss of protein in urine.

• (b) Acute hepatitis → Albumin decreases because the liver’s synthetic function is impaired.

• (c) Chronic inflammation → Albumin decreases as it is a negative acute-phase reactant.

The first step in analyzing a 24-hour urine specimen for quantitative protein is to measure the total volume accurately. This is essential because the protein concentration (e.g., mg/dL) must be multiplied by the total volume to calculate the total protein excretion over 24 hours (reported as mg/24 hours).

• a) Subculture the urine → not necessary for protein quantification.

• b) Add preservative → should be done during collection, not as the first step in analysis.

• c) Screen with dipstick → may be done but is not the first step in the quantitative procedure.

• Haptoglobin is a plasma glycoprotein that binds free hemoglobin released into the blood during hemolysis. This binding prevents kidney damage and allows the complex to be cleared by the reticuloendothelial system.

Other options:

• a) Ceruloplasmin → Binds copper, not hemoglobin.

• b) Transferrin → Binds and transports iron in plasma.

• d) Fibrinogen → Involved in blood clotting, not hemoglobin binding.

Albumin is the most abundant plasma protein in normal human serum, typically comprising about 55–65% of total serum protein, with a concentration of about 3.5–5.0 g/dL.

Other options

• a) Globulin: Group of various proteins (immunoglobulins, transport proteins) — less abundant than albumin
• c) Fibrinogen: Present in plasma, absent in serum; ~0.2–0.4 g/dL
• d) Transferrin:Iron transport protein; only a small fraction of total proteins

Amniotic fluid alpha-fetoprotein (AFP) measurement is a key laboratory test used in the prenatal diagnosis of open neural tube defects (such as spina bifida and anencephaly).

• In open neural tube defects, AFP leaks from the fetal circulation into the amniotic fluid, leading to elevated levels.

• This is often combined with maternal serum AFP screening and confirmed by ultrasound.

Other options:

• (b) Amniotic fluid estriol → Used to assess fetal well-being and placental function, not NTDs.

• (c) Maternal serum estradiol → Monitors ovarian and placental function, not NTDs.

• (d) Maternal serum estrone → Also reflects placental estrogen production, not neural tube integrity.

Essential amino acids are those that cannot be synthesized by the human body in sufficient amounts and must be obtained from the diet.

Other options (nonessential amino acids):

• (a) Glycine → Nonessential (synthesized from serine)

• (b) Alanine → Nonessential (formed from pyruvate)

• (d) Tyrosine → Nonessential if phenylalanine is available (it’s synthesized from phenylalanine)

• The Kjeldahl method is a classical technique for determining total protein in a sample.

• Proteins contain nitrogen, so the method measures total nitrogen and then converts it to protein content using a standard factor (usually 6.25, assuming proteins are ~16% nitrogen).

Other options:

• b) Peptide bonds → Measured by Biuret or Lowry methods.

• c) Copper binding → Biuret method.

• d) Absorbance at 280 nm → Measures proteins containing aromatic amino acids (tryptophan, tyrosine).

Bence Jones proteins are free monoclonal immunoglobulin light chains in the urine, typically associated with plasma cell dyscrasias like multiple myeloma.

• Immunofixation electrophoresis (IFE) is the most specific and sensitive method for identifying and typing these monoclonal proteins.

• Other methods:

  • a) Sulfosalicylic acid test → detects total protein, not specific for Bence Jones.

  • b) Urine reagent strips → primarily detect albumin, not Bence Jones proteins.

  • d) Biuret reaction → measures total protein, not specific.

Bence Jones proteins are free monoclonal light chains of immunoglobulins. They are produced in excess by malignant plasma cells in multiple myeloma. These proteins are small enough to be filtered by the kidneys and are most often detected in the urine rather than serum. Detection in urine is an important diagnostic marker for multiple myeloma.

• b) Serum of diabetic patients → Incorrect; diabetes is not associated with Bence Jones proteins.

• c) CSF of meningitis patients → Incorrect; Bence Jones proteins are not related to infections.

• d) Plasma of anemic patients → Incorrect; anemia does not cause Bence Jones proteins.

Fibrinogen is a soluble plasma protein that is converted into fibrin by the enzyme thrombin during the blood clotting process, forming the meshwork of a blood clot.

Other options 

• (b) Albumin → Maintains osmotic pressure and transports molecules.

• (c) Transferrin → Transports iron in the blood.

• (d) Haptoglobin → Binds free hemoglobin to prevent renal loss and oxidative damage.

In glomerulonephritis, damage to the glomeruli reduces the kidney’s ability to filter waste products from the blood. This leads to the retention of nitrogenous wastes, resulting in an increased Blood Urea Nitrogen (BUN).

The other options are not typical:

• a) Creatinine decreased → No, creatinine is usually increased due to reduced clearance.

• b) Calcium increased → Not characteristic; hypocalcemia may occur in chronic kidney disease.

• c) Phosphorous decreased → No, phosphorus is often increased due to reduced renal excretion

Urea is the primary end product of protein and amino acid (nitrogen) metabolism in humans. It is synthesized in the liver via the urea cycle and excreted by the kidneys, accounting for the majority of nitrogenous waste in urine.

Other options:

• (a) Amino acids → Used for protein synthesis or energy, not an excretory form.

• (b) Creatinine → Product of muscle metabolism (from creatine phosphate), not the main nitrogen waste.

• (d) Uric acid → End product of purine metabolism, not general nitrogen metabolism.

• Ceruloplasmin is a copper-binding glycoprotein in plasma that transports the majority of copper in the bloodstream.

• It also has ferroxidase activity, helping convert Fe²⁺ to Fe³⁺ for binding to transferrin.

Other options:

• a) Albumin → Transports many small molecules, including some metals, but not the main copper carrier.

• c) Haptoglobin → Binds free hemoglobin.

• d) Transferrin → Transports iron, not copper.

Denaturation is the process in which a protein loses its secondary, tertiary, or quaternary structure due to disruption of non-covalent interactions (such as hydrogen bonds, hydrophobic interactions, and disulfide bridges) while the primary structure (peptide bonds) remains intact.

Other options (incorrect):

• (a) Breaking peptide bonds → That’s hydrolysis, not denaturation.

• (c) Hydrolysis into amino acids → Complete breakdown of protein (different process).

• (d) Conversion into nucleic acids → Biochemically unrelated.

• Immunoglobulins (antibodies) are plasma proteins that play a key role in the immune response.

• In serum protein electrophoresis, they are mainly found in the gamma-globulin fraction.

Other options:

• a) Alpha-globulins → Transport proteins and some protease inhibitors.

• b) Beta-globulins → Transport proteins like transferrin, some complement proteins.

• d) Albumin → Maintains oncotic pressure and transports small molecules; not antibodies.

A pleural effusion caused by Streptococcus pneumoniae is typically an exudative effusion (due to infection/inflammation).

According to Light’s criteria, an effusion is considered exudative if any of the following are true:

1. Pleural fluid protein / serum protein > 0.5

2. Pleural fluid LDH / serum LDH > 0.6

3. Pleural fluid LDH > 2/3 of the upper limit of normal serum LDH

Albumin is the most abundant plasma protein, accounting for about 60% of total serum protein, and it is synthesized exclusively by the liver.

Other options (incorrect):

• (a) Pancreas → Produces digestive enzymes (amylase, lipase, etc.).

• (c) Kidney → Filters proteins; does not synthesize albumin.

• (d) Small intestine → Site of protein digestion and absorption, not synthesis.

The normal reference range for total serum protein in adults is approximately:

$$\text{6.0 – 8.0 g/dL (60 – 80 g/L)}$$

• ↑ Total protein:

  • Dehydration

  • Chronic inflammation

  • Multiple myeloma (↑ immunoglobulins)

• ↓ Total protein:

  • Liver disease (↓ synthesis)

  • Nephrotic syndrome (loss via urine)

  • Malnutrition or malabsorption

The Biuret method relies on the reaction of copper ions in an alkaline solution with the peptide bonds in proteins, producing a violet-colored complex. The intensity of the color is proportional to the protein concentration.

Other options (incorrect):

• (a) Binding of a dye → Used in albumin assays (e.g., bromocresol green/purple methods).

• (c) Reaction with ninhydrin → Used for amino acid detection, not total protein.

• (d) Reduction of copper ions → Happens as part of the reaction, but the color formation depends specifically on peptide bonds, not on simple reduction.