Clinical Aspects of Biochemistry Proteins and Disease Blood clotting enzymes

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