Short not on plasma proteins(its values) and their functions and write a short note on A/G ratio for 5 marks
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Plasma Proteins - Values and Functions
Normal Values
| Protein Fraction | Normal Value |
|---|---|
| Total plasma protein | 6.0 - 8.0 g/dL |
| Albumin | 3.5 - 5.0 g/dL (~60% of total) |
| Globulins (total) | 2.0 - 3.5 g/dL |
| - α1-globulins | 0.2 - 0.4 g/dL |
| - α2-globulins | 0.5 - 0.9 g/dL |
| - β-globulins | 0.6 - 1.1 g/dL |
| - γ-globulins | 0.7 - 1.7 g/dL |
| Fibrinogen | 0.2 - 0.4 g/dL |
Functions of Plasma Proteins
1. Oncotic (Osmotic) Pressure Maintenance
Plasma proteins, mainly albumin, exert an osmotic (oncotic) pressure of ~25 mm Hg across the capillary wall. This draws fluid back into the capillaries at the venous end and prevents edema. Albumin alone accounts for 70-80% of this osmotic pressure despite contributing ~60% of total protein by weight.
2. Transport / Carrier Function
Plasma proteins carry many insoluble or poorly soluble substances:
- Albumin: carries free fatty acids, bilirubin, calcium, zinc, copper, steroid hormones, and numerous drugs (e.g. penicillin, aspirin, warfarin)
- Transferrin: transports iron
- Ceruloplasmin: transports copper
- Haptoglobin: binds free hemoglobin
- Thyroxine-binding globulin (TBG): transports thyroid hormones
- Transcortin (CBG): transports cortisol and other corticosteroids
- Transthyretin: transports T4 and retinol-binding protein
3. Immune Defense
Immunoglobulins (IgG, IgA, IgM, IgD, IgE), found in the γ-globulin fraction, are antibodies synthesized by lymphocytes. Complement proteins also serve in innate immunity.
4. Blood Coagulation
Fibrinogen (Factor I) and all coagulation factors except Factor VIII are plasma proteins synthesized by the liver. They are essential for hemostasis and fibrinolysis (e.g., plasminogen).
5. Buffering Capacity
Plasma proteins contribute ~15% of the total buffering capacity of the blood due to the ionizable -COOH and -NH2 groups. At pH 7.4, they are predominantly in anionic form.
6. Acute Phase Response
Proteins like C-reactive protein (CRP), α1-antitrypsin, haptoglobin, and α1-acid glycoprotein are acute phase proteins that rise rapidly during inflammation and tissue damage.
7. Enzyme Activity
Some plasma proteins function as enzymes within the blood (e.g., coagulation factors, cholinesterase) or "leak" from damaged tissues into blood (e.g., aminotransferases).
A/G Ratio (Albumin to Globulin Ratio)
Definition
The A/G ratio is the ratio of serum albumin to total serum globulin.
Globulin = Total protein - Albumin
A/G ratio = Albumin / Globulin
Normal Value
1.2 : 1 to 1.8 : 1 (generally stated as >1, i.e., albumin normally exceeds globulin)
Significance
The A/G ratio is clinically useful because disease states typically depress albumin and raise globulins simultaneously, causing an exaggerated shift in the ratio (numerator falls while denominator rises).
| Condition | Effect on A/G Ratio |
|---|---|
| Liver disease (cirrhosis, hepatitis) | Decreased (albumin synthesis falls) |
| Nephrotic syndrome | Decreased (urinary albumin loss) |
| Malnutrition / Malabsorption | Decreased (albumin synthesis falls) |
| Multiple myeloma | Reversed (marked globulin rise) |
| Autoimmune hepatitis | Reversed (polyclonal IgG rises) |
| Primary biliary cirrhosis | Reversed (polyclonal IgM rises) |
| Kala-azar (visceral leishmaniasis) | Reversed (greatly increased IgG) |
| Normal / Well-nourished state | >1 |
Why is Reversal Significant?
A reversed A/G ratio (<1) indicates either decreased albumin synthesis, increased albumin loss, or increased immunoglobulin production. Albumin synthesis decreases early in protein malnutrition (kwashiorkor) and liver disease, making the A/G ratio a sensitive marker of these conditions.
Note: While the A/G ratio was widely used historically, serum protein electrophoresis (SPEP) is now more informative as it separates the five major fractions (albumin, α1, α2, β, γ) individually and can identify a monoclonal spike in conditions like myeloma.
Sources: Ganong's Review of Medical Physiology (26e); Harper's Illustrated Biochemistry (32e); Basic Medical Biochemistry - A Clinical Approach (6e); Henry's Clinical Diagnosis and Management by Laboratory Methods
For mbbs first year
Here is the same content rewritten for MBBS 1st year (Biochemistry level) - concise, exam-focused, and easy to write in 5 marks:
Plasma Proteins - Values, Functions & A/G Ratio
(5 Marks)
Plasma Proteins - Normal Values
| Fraction | Normal Value |
|---|---|
| Total Plasma Protein | 6.0 - 8.0 g/dL |
| Albumin | 3.5 - 5.0 g/dL |
| Globulins | 2.0 - 3.5 g/dL |
| Fibrinogen | 0.2 - 0.4 g/dL |
- Albumin makes up ~60% of total plasma protein
- Site of synthesis: Liver (except immunoglobulins, which are made by plasma cells/B lymphocytes)
Functions of Plasma Proteins
1. Maintenance of Osmotic (Oncotic) Pressure
Plasma proteins, mainly albumin, maintain a colloid osmotic pressure of ~25 mm Hg. This prevents leakage of fluid into tissues. Loss of albumin → edema (e.g., nephrotic syndrome, kwashiorkor).
2. Transport Function
Proteins carry insoluble substances in blood:
- Albumin - free fatty acids, bilirubin, Ca²⁺, drugs (aspirin, penicillin, warfarin)
- Transferrin - iron
- Ceruloplasmin - copper
- Haptoglobin - free hemoglobin
- TBG (Thyroxine Binding Globulin) - thyroid hormones
- Transcortin - cortisol
3. Role in Blood Coagulation
Fibrinogen and all clotting factors (except Factor VIII) are plasma proteins synthesized in the liver. Essential for hemostasis.
4. Immune Defense
Immunoglobulins (IgG, IgA, IgM, IgD, IgE) in the γ-globulin fraction act as antibodies. Complement proteins aid in innate immunity.
5. Buffer Action
Plasma proteins act as buffers due to ionizable -COOH and -NH₂ groups. They contribute ~15% of buffering capacity of blood.
6. Acute Phase Proteins
During inflammation, liver increases synthesis of CRP (C-reactive protein), α1-antitrypsin, haptoglobin - these are acute phase reactants.
A/G Ratio (Albumin : Globulin Ratio)
Formula:
Globulin = Total protein - Albumin A/G ratio = Albumin ÷ Globulin
Normal value: 1.2 to 1.8 : 1 (albumin is always MORE than globulin normally)
Clinical Significance
In disease, albumin falls and globulins rise simultaneously → A/G ratio decreases or gets "reversed" (becomes <1).
| Condition | Change in A/G ratio |
|---|---|
| Liver cirrhosis / hepatitis | Decreased |
| Nephrotic syndrome | Decreased (albumin lost in urine) |
| Protein malnutrition (Kwashiorkor) | Decreased |
| Multiple myeloma | Reversed (massive globulin rise) |
| Kala-azar | Reversed |
| Autoimmune hepatitis | Reversed |
Why is it useful?
Because both albumin and globulin move in opposite directions in disease, the ratio amplifies the abnormality - making it a sensitive indicator of diseases like liver failure, nephrotic syndrome, and myeloma.
Remember: Normal = A > G. Reversed = G > A (pathological).
Key one-liners to remember:
- Total protein = 6-8 g/dL; Albumin = 3.5-5 g/dL
- Albumin = main osmotic protein + main transport protein
- A/G ratio normal = 1.2-1.8; reversed in liver disease, nephrotic syndrome, myeloma
- Fibrinogen is removed in serum (so serum has no fibrinogen; plasma does)
Immunoglobulins structure types and functions 5 marks mbbs
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Immunoglobulins - Structure, Types and Functions
(5 Marks - MBBS 1st Year)
Definition
Immunoglobulins (Ig) are glycoproteins produced by plasma cells (differentiated B lymphocytes) that function as antibodies. They are found in the γ-globulin fraction of plasma proteins.
Basic Structure of Immunoglobulin (IgG as prototype)
All immunoglobulins are built on a basic four-chain unit (monomer):
Chains
- 2 Heavy (H) chains - larger (~50-70 kDa each)
- 2 Light (L) chains - smaller (~23 kDa each)
- Held together by inter-chain disulfide bonds (-S-S-) and non-covalent forces
Regions on each chain
Each chain has:
- Variable (V) region - N-terminal end; differs between antibodies; forms the antigen-binding site
- Constant (C) region - C-terminal end; same within a class; determines effector functions
Light Chains
Two types: Kappa (κ) and Lambda (λ)
- A single Ig molecule always has either 2κ or 2λ, never mixed
- In humans, κ chains predominate
Heavy Chains
Each class has a distinct heavy chain:
| Class | Heavy chain |
|---|---|
| IgG | γ (gamma) |
| IgA | α (alpha) |
| IgM | μ (mu) |
| IgD | δ (delta) |
| IgE | ε (epsilon) |
Functional Fragments (Proteolytic Cleavage)
| Enzyme | Fragments Produced |
|---|---|
| Papain | 2 × Fab (Fragment Antigen Binding - each monovalent) + 1 × Fc (Fragment Crystallizable) |
| Pepsin | 1 × F(ab')₂ (bivalent, still precipitates antigen) + pFc' (degraded) |
- Fab - contains VH + VL + CH1 + CL; binds antigen
- Fc - contains CH2 + CH3; mediates effector functions (complement, opsonization, placental transfer)
- Hinge region - between CH1 and CH2; flexible; susceptible to proteases
Types of Immunoglobulins and Their Properties
| Property | IgG | IgA | IgM | IgD | IgE |
|---|---|---|---|---|---|
| % of total Ig | 75% | 15% | 9% | 0.2% | 0.004% |
| Serum conc. | ~1000 mg/dL | ~200 mg/dL | ~120 mg/dL | ~3 mg/dL | ~0.05 mg/dL |
| Structure | Monomer | Monomer/Dimer | Pentamer | Monomer | Monomer |
| Mol. weight | 150 kDa | 170/400 kDa | 900 kDa | 180 kDa | 190 kDa |
| Sedimentation | 7S | 7S or 11S | 19S | 7S | 8S |
Functions of Each Class
IgG (most abundant)
- Main antibody of secondary immune response
- Crosses the placenta - provides passive immunity to the newborn
- Opsonization - coats bacteria to enhance phagocytosis
- Complement activation (classical pathway) - IgG1 and IgG3 most effective
- Neutralization of toxins and viruses
- Has 4 subclasses: IgG1, IgG2, IgG3, IgG4
IgA
- Found in secretions (saliva, tears, breast milk, gut, respiratory mucosa) as secretory IgA (sIgA)
- Dimer in secretions, joined by J chain + secretory component (SC)
- First line of mucosal defense against pathogens
- Protects mucosal surfaces (GIT, respiratory tract)
IgM
- First antibody produced in primary immune response (earliest response)
- Pentameric structure - 10 antigen binding sites (highest avidity)
- Most efficient at complement activation
- Antigen receptor on naive B cells (as surface monomer)
- Responsible for ABO blood group agglutination (isohemagglutinins)
- Contains J chain linking the 5 monomers
IgD
- Found mainly on the surface of B lymphocytes as antigen receptor (along with IgM)
- Very low serum levels
- Role in lymphocyte activation and regulation
- May protect against upper respiratory tract microbes
IgE
- Present in extremely low concentrations in serum
- Most IgE is bound to mast cells and basophils via high-affinity Fc receptors (FcεRI)
- When antigen (allergen) cross-links IgE on mast cells → degranulation → histamine release
- Mediates Type I hypersensitivity (allergic reactions) - anaphylaxis, asthma, urticaria
- Plays a role in defense against helminth (worm) infections
Key Points to Remember
| Feature | Remember |
|---|---|
| Most abundant | IgG (75%) |
| First in infection | IgM |
| Crosses placenta | IgG only |
| Found in secretions | IgA |
| Allergy/anaphylaxis | IgE |
| B cell receptor | IgM and IgD |
| Pentamer | IgM |
| Highest avidity | IgM (10 binding sites) |
Sources: Roitt's Essential Immunology; Harper's Illustrated Biochemistry (32e); Medical Microbiology 9e
Mention functions and types
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Here is a focused, exam-ready summary of just the types and functions of immunoglobulins - clean and easy to write in an answer:
Immunoglobulins - Types and Functions
(MBBS 1st Year)
Types of Immunoglobulins (5 Classes)
There are 5 classes of immunoglobulins based on the heavy chain type:
| Class | Heavy Chain | Structure | % in Serum |
|---|---|---|---|
| IgG | γ (gamma) | Monomer | 75% |
| IgA | α (alpha) | Monomer / Dimer | 15% |
| IgM | μ (mu) | Pentamer | 9% |
| IgD | δ (delta) | Monomer | 0.2% |
| IgE | ε (epsilon) | Monomer | 0.004% |
Light chains are of 2 types: Kappa (κ) and Lambda (λ) - same in all classes.
Functions of Each Type
IgG
- Main antibody of secondary (anamnestic) immune response
- Only Ig that crosses the placenta - provides passive immunity to newborn
- Opsonization - coats bacteria, enhances phagocytosis
- Complement activation (classical pathway)
- Neutralizes toxins and viruses
- Has 4 subclasses: IgG1, IgG2, IgG3, IgG4
IgA
- Mucosal/secretory immunity - main Ig in saliva, tears, breast milk, colostrum, respiratory and GI secretions
- In secretions, exists as dimer joined by J chain + secretory component (SC)
- First line of defense at mucosal surfaces
- Protects gut and respiratory tract from pathogen entry
IgM
- First antibody produced in primary immune response (earliest to appear)
- Pentameric - has 10 antigen binding sites (highest avidity of all Ig)
- Most potent activator of complement (classical pathway)
- Acts as antigen receptor on naive B cells (as surface monomer)
- Responsible for ABO blood group agglutination (natural isohemagglutinins)
IgD
- Found mainly on surface of mature B lymphocytes as antigen receptor (co-expressed with IgM)
- Involved in B cell activation and regulation
- Very low serum levels - exact serum function not fully clear
- May have a role in defense against upper respiratory infections
IgE
- Mediates Type I (immediate) hypersensitivity reactions - allergy, asthma, anaphylaxis, urticaria
- Binds to mast cells and basophils via high-affinity Fc receptors (FcεRI)
- When allergen cross-links IgE → mast cell degranulation → histamine, leukotrienes released
- Plays a protective role in helminth (parasitic worm) infections
- Lowest serum concentration but most potent in allergic reactions
Quick Memory Table
| Ig | Key Function - One Line |
|---|---|
| IgG | Most abundant; crosses placenta; opsonization |
| IgA | Secretory; mucosal protection |
| IgM | First responder; strongest complement activator; pentamer |
| IgD | B cell surface receptor; lymphocyte regulation |
| IgE | Allergy and anaphylaxis; anti-helminth |
Mnemonic for order of serum concentration (high to low):
G A M D E → "Go And Meet Doctor Edelman"
Structure of insulin and its maturation
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Structure of Insulin and Its Maturation
(MBBS 1st Year - Biochemistry)
Structure of Insulin
Insulin is a small polypeptide hormone with the following structural features:
Chains
| Feature | Detail |
|---|---|
| A chain | 21 amino acids |
| B chain | 30 amino acids |
| Total | 51 amino acids, mol. wt ~5808 Da |
| Gene location | Short arm of chromosome 11 |
Disulfide Bonds (3 total)
- 2 interchain disulfide bonds - link A chain to B chain (at A7-B7 and A20-B19)
- 1 intrachain disulfide bond - within the A chain itself (at A6-A11)
These disulfide bonds are essential for the correct 3D shape and biological activity of insulin.
Maturation (Biosynthesis) of Insulin
Insulin is synthesized in the β-cells of the Islets of Langerhans in the pancreas. It goes through 3 precursor stages before becoming active:
Step-by-Step Maturation
Gene (Chromosome 11)
↓ Transcription + Translation on ribosomes of RER
PREPROINSULIN (110 amino acids)
↓ Signal peptide cleaved in RER lumen
PROINSULIN (86 amino acids)
↓ Disulfide bonds form → correct folding
↓ Transferred to Golgi → packaged into secretory granules
↓ Proteases cleave C-peptide (+ 2 dipeptides)
ACTIVE INSULIN (51 amino acids) + C-PEPTIDE
↓ Exocytosis (triggered by ↑ blood glucose)
BLOODSTREAM
1. Preproinsulin
- First product of translation on RER ribosomes
- Contains: Signal peptide (24 aa) + B chain + C peptide + A chain
- The hydrophobic signal peptide directs the polypeptide into the lumen of the RER and is immediately cleaved off
2. Proinsulin
- Formed after signal peptide removal (86 amino acids)
- Folds into correct conformation in the RER
- 3 disulfide bonds form between cysteine residues
- Transferred to Golgi apparatus → packaged into secretory granules
- Biologically inactive
3. Active Insulin + C-Peptide
- Inside secretory granules, two proteases (Prohormone convertase 1 and 2) cleave the C-peptide (connecting peptide, 31 amino acids) + 2 small dipeptides
- Result: Active insulin (A + B chains held by disulfide bonds) + C-peptide
- Released in equimolar amounts by exocytosis
- Zinc ions co-precipitate with insulin in granules → form hexameric zinc-insulin crystals for storage
Clinical Significance of C-Peptide
| Point | Detail |
|---|---|
| Released equimolar with insulin | Yes - 1:1 ratio |
| Present in exogenous (injected) insulin? | No - commercial insulin has no C-peptide |
| Use | Measures endogenous β-cell function in diabetics receiving insulin injections |
| Half-life | Longer than insulin (~30 min vs ~5 min) |
C-peptide measurement is used to distinguish Type 1 DM (low/absent C-peptide) from Type 2 DM (normal/high C-peptide) and to detect insulinoma.
Summary Table
| Precursor | Location | What happens |
|---|---|---|
| Preproinsulin | RER (ribosomes) | Signal peptide cleaved |
| Proinsulin | RER → Golgi | Disulfide bonds form; folding |
| Active Insulin | Secretory granules | C-peptide removed by proteases |
| Secretion | Plasma membrane | Exocytosis triggered by ↑ glucose |
Key facts to remember:
- A chain = 21 aa; B chain = 30 aa; Total = 51 aa
- 3 disulfide bonds (2 interchain, 1 intrachain)
- Order: Preproinsulin → Proinsulin → Insulin
- C-peptide is released equimolar with insulin
- Gene on chromosome 11
- Half-life of insulin in circulation = ~5 minutes
Sources: Costanzo Physiology (7e); Ganong's Review of Medical Physiology (26e); Basic Medical Biochemistry - A Clinical Approach (6e)
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