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Plasma Proteins
Prof Dr Kwan Teck Kim
Department of Biochemistry
Faculty of Medicine
Mahsa 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 proteins
 Electrophoresis
 Albumin, functions, clinical significance
 Hypo-albuminemia
 Globulins, alpha, beta, gamma
 Transport proteins in blood
 Acute phase proteins in blood
 Ceruloplasmin
 Alpha-1 anti-trypsin
 Clotting 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 Albumin
1.
i.
Colloid Osmotic Pressure of Plasma
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 Function
i. Bilirubin & non-esterified fatty acids are specifically
transported by albumin.
ii. Drugs (supha, aspirin, salicylates, dicoumarol, pheytoin)
iii. Hormones (Steroid hormones, thyroxine)
iv. 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 Proteins
1. Albumin: chief stabiliser of blood volume and helps in
transport of drugs, bilirubin, free fatty acids, calcium,
hormones & metals
serves as a source amino acids for tissue protein
synthesis
helps 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 proteins
1. C-Reactive Protein (CRP)
β-globulin, 115-140 kD,
stimulates complement activity & macrophage
phagocytosis
has 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 1000
 Bacterial 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 proteases
 Important in vivo anti-coagulant
 Carrier of many growth factors such as platelet derived
growth factor (PDGF).
 Normal serum level is 130-300 mg/dl
 Its concentration is markedly increased (up to 2-3
g/dl) in Nephrotic syndrome
Clotting Factors
1. Prothrombin
 Single chain zymogen, 69 kD
 Plasma concentration 10-15 mg/dl
 Prothrombin → thrombin by Factor Xa, by removal
of N-terminal fragment
2. Thrombin
 Serine protease, 34 kD
 The 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.