Clinical Aspects of Biochemistry

Proteins and Disease

Blood clotting enzymes

Blood clotting

1. Platelet aggregation

2. Constriction of injured vessels

3. Fibrin formation

THE CLOTTING CASCADE - A SIMPLE VIEW

FIBRIN FIBRE

FIBRINOGEN - FIBRIN CONVERSION

FIBRINOGEN

~450ÅFrom Voigt & Voigt

FIBRINOGEN - POLYPEPTIDE CHAIN ORGANIZATION

From Voigt & Voigt

FIBRIN AGGREGATION

From Voigt & Voigt

Transamidase (Factor XIIIa)

Gln

Lys

CROSSLINKING FIBRIN

From Stryer

-carboxy glutamic acid (Gla):• ~10 residues near N-terminus of prothrombin• also found in other enzymes of clotting cascade• synthesis requires vitamin K (antagonised by dicoumarol and warfarin)• facilitates binding to Ca2+ and membrane localisation

PROTHROMBIN

Factor X (Stuart factor)• Glycoprotein. Mr 55,000• L chain (16.5K) and H chain (39K) joined by S-S bridge. ‘Pro’ bit stays attached after activation• Activated by cleavage of 51 residues from N-terminus of H chain by factor IXa + factor VIII + Ca2+ + phospholipid or factor VIIa + tissue factor

Factor IX (Christmas factor)• Glycoprotein. Mr 55,000• ~ 15% of haemophilia• Activated by removal of 11K fragment from factor XIa:

27K

S S

17K

11KIXa:

FACTOR VIII (antihaemophilia factor)

Lack of factor VIII causes haemophilia A - 70-80% of all haemophiliaProtein of 2351 aas; gene ~186,000 bp:.

From Stryer

CONTROL OF CLOTTING

1. Dilution and localisation

2. Antithrombin (58K protein; a serpin)

3. Heparin (sulphated glycosaminoglycan)

4. Protein C (zymogen; activated by thrombin)

5. Thrombomodulin (74K glycoprotein)

[hirudin - leeches]

INTRINSIC PATHWAY - FEEDBACK

CLOT LYSIS

Plasmin:• a serine protease (trypsin like) which specifically breaks down fibrin• formed from plasminogen (86K protein), by action of other proteases : urokinase (kidney) or tissue plasminogen activator (tPA)

tPA:

From Stryer

From Stryer