Plasma Proteins

Prof Dr Kwan Teck KimDepartment of Biochemistry

Faculty of MedicineMahsa University College

Learning outcomes

On completion of this topic, the student will be able to:

Explain the basis of classification of plasma proteins

Name the major types of plasma proteins

Discuss the major roles of plasma proteins

Contents

Plasma proteinsElectrophoresisAlbumin, functions, clinical

significanceHypo-albuminemiaGlobulins, alpha, beta, gammaTransport proteins in bloodAcute phase proteins in bloodCeruloplasminAlpha-1 anti-trypsinClotting factors

Protein molecules in blood

Electrophoresis

Employed for separation of serum proteins

Movement of charged particles through an electrolyte when subjected to an electric field

Positively charged particles (cations) move to cathode

Negatively charged particles (anions) move to anode

At pH 8.6 (barbitone buffer), serum proteins will have net negative charge and migrate towards anode.

Electrophoresis (contd)

• In agar gel electrophoresis, normal serum is separated into 5 bands:

• Albumin : 55-65%• Alpha-1 globulin : 2-4%• Alpha-2 globulin : 6-12%• Beta-globulin : 8-12%• Gamma-globulin : 12-22%Albumin has the maximum and gamma globulin has

the minimum mobility in the electrical field. γ- globulins contain the antibodies (immunoglobulins),Most of α-1 fraction is made up of α-1-antitrypsin; α-2 band is mainly made up by α-2-macroglobulin. β-fraction contains low density lipoproteins.

Anode

Cathode

Electrophoretogram

Functions of Albumin1. Colloid Osmotic Pressure of Plasmai. Proteins cannot easily escape out of blood vessels, &

therefore, proteins exert the ‘effective osmotic pressure’. It is about 25 mm Hg, & 80% of it is contributed by albumin.

ii. According to Starling’s hypothesis, at the capillary end the BP or hydrostatic pressure expels water out, & effective osmotic pressure (EOP) takes water into the vascular compartment.

iii. At the arterial end of capillary, BP is 35 mm Hg & EOP 25 mm; thus water is expelled by a pressure of 10mm Hg.

iv. At the venous end of capillary, EOP is 25 mm and BP is 15 mm, water is imbibed with a pressure of 10 mm.

Albumin (contd 1)

v. If protein concn in serum is reduced, the EOP is correspondingly decreased. Then return of water into blood vessels is diminished, leading to accumulation of water in tissues (edema).

2. Transport Functionvi. Bilirubin & non-esterified fatty acids are

specifically transported by albumin.vii. Drugs (supha, aspirin, salicylates, dicoumarol,

pheytoin)viii.Hormones (Steroid hormones, thyroxine)ix. Metals (Ca, Cu) and heavy metals are

nonspecifically carried by albumin.

Albumin (contd 2)

3. Nutritional Function

All tissue cells can take up albumin by pinocytosis.It is then broken down to amino acid level.

So, albumin may be considered as the transport form of essential amino acids from liver to other tissues.

Plasma Proteins

Total blood volume is ~ 4.5 – 5 litres Defibrinated plasma is called serum

Total protein of normal plasma, 6-8 g/100 ml

Consists of 3.5-5.0 g/dl albumin; 2.5-3.5 g/dl globulins and 200-400 mg/dl fibrinogen

Plasma proteins (contd)

The albumin : globulin ratio, 1.2 : 1 to 1.5 : 1

Hypo-albuminemia is seen in cirrhosis, chronic liver failure, malnutrition, malabsorption diseases, nephrotic syndrome, burns

All (except immunoglobulins) are synthesized in liver; mostly glycoproteins

Abnormal Patterns in Clinical Diseases

1. Chronic infections: The γ globulins are increased, but increase is smooth and wide-based

2. Multiple myeloma: In para-proteinemias, a sharp spike termed M-Band.

3. Nephrotic syndrome: All proteins except very big molecules are lost through urine. Alpha-2 fraction (containing macroglobulin) is very prominent

Functions of Plasma Proteins1. Albumin: chief stabiliser of blood volume and

helps in transport of drugs, bilirubin, free fatty acids, calcium, hormones & metalsserves as a source amino acids for tissue protein synthesishelps in maintaining colloidal osmotic pressure; buffering capacity (see earlier slides no. 11-13)

2. Gobulins:

α 1 –globulin (retinol binding protein)

involved in transport of vitamin A

α 2 –globulin (ceruloplasmin) involved in

transport of copper

β-globulin (transferrin) an iron transport protein

γ-globulins (antibodies) confer immunity against infectious diseases

Fibrinogen: Participates in blood coagulation

Acute phase proteins1. C-Reactive Protein (CRP)

β-globulin, 115-140 kD, stimulates complement activity & macrophage phagocytosishas a positive correlation in predicting the risk of coronary artery diseases

2. Ceruloplasmin

α2-globulin, 160 kD; 6-8 copper atoms per molecule;also called Ferroxidase which helps in incorporation of Fe into transferrin.

An important anti-oxidant in plasma

2. Ceruloplasmin (contd) 90% Cu in plasma is bound to

ceruloplasmin 10% Cu bound with albumin loosely Lower level of ceruloplasmin in

Wilson’s disease, malnutrition, nephrosis, cirrhosis

Increased level in active hepatitis, biliary cirrhosis, hemochromatosis, obstructive biliary disease

3. Alpha-1 Anti-trypsin (AAT)• Also called Alpha-anti-proteinase or

protease inhibitor• It inhibits all serine proteases (proteolytic

enzymes having a serine at their active centre), such as plasmin, thrombin, trypsin, chymotrypsin, elastase, cathepsin.

• Serine protease inhibitors (Serpins)• AAT is synthesized in liver. A glycoprotein,

50 kD. • Normal serum level 75-200 mg/dl• AAT deficiency causes Emphysema and

Nephrotic syndrome.

3. Alpha-1 Anti-trypsin (AAT) [contd]

Emphysema: AAT deficiency 1 in 1000Bacterial infections in lung attract macrophages

which release elastase.In AAT deficiency, unopposed action of elastase

causes damage to lung tissue, leading to emphysema.

About 5% of emphysema cases are due to AAT deficiency

Nephrotic syndrome:• AAT molecules are lost in urine• Hence, AAT deficiency is produced

Emphysema: AAT deficiency 1 in 1000Bacterial infections in lung attract macrophages

which release elastase.In AAT deficiency, unopposed action of elastase

causes damage to lung tissue, leading to emphysema.

About 5% of emphysema cases are due to AAT deficiency

Nephrotic syndrome:• AAT molecules are lost in urine• Hence, AAT deficiency is produced

4. Alpha-2-Macroglobulin (AMG)AMG, tetrameric protein, 725 kD

Major component of α 2 –globulins

Synthesized by hepatocytes and macrophages

Inactivates all proteasesImportant in vivo anti-coagulantCarrier of many growth factors such as

platelet derived growth factor (PDGF).Normal serum level is 130-300 mg/dlIts concentration is markedly increased (up

to 2-3 g/dl) in Nephrotic syndrome

Clotting Factors1. ProthrombinSingle chain zymogen, 69 kDPlasma concentration 10-15 mg/dlProthrombin → thrombin by Factor Xa, by

removal of N-terminal fragment

2. ThrombinSerine protease, 34 kDThe Ca2+ binding of prothrombin is essential for

anchoring prothrombin on surface of platelets.When terminal fragment is cleaved off, Ca2+ binding

sites are removed.So, thrombin is released from platelet surface.

Clotting Factors (contd)

3. Fibrinogen An acute phase protein Synthesized by liver, 340 kD Normal fibrinogen level in blood, 200-400

mg/dl Fibrinogen → fibrin by cleaving Arg-Gly

peptide bonds of fibrinogen Fibrin monomers formed are insoluble They align themselves lengthwise, aggregate

and precipitate to form the clot

References

• Devlin T.M. [Ed.] (2006), Textbook of Biochemistry with Clinical Correlations, 6th Ed., Wiley-Liss, Inc, N.Y.

Vasudeven, D.M., Sreekumari, S. & Vaidyanathan, K. (2011). Textbook of Biochemistry for Medical Students , 6th Ed., Jaypee Brothrs Medical Publishers Ltd, New Delhi.

Murray, R.K. et al. (2009) Harper’s Illustrated Biochemistry, 28th Ed., Mc Graw Hill Medical, N.Y.