User:Radhikabhake/sandbox

In this article, I would like to add gene structure, mechanism of action and function of gene.

Staphylokinase (SAK) is an amino acid enzyme from Staphylococcus aureus Staphylokinase is a 136 amino acid bacteriophage which is a 15KDa protein. Staphylokinase contains 136 amino acid residues and is a 15KDa protein. Growth of Staphylokinase is seen in late exponential phase.

It is positively regulated by the "agr" gene regulator. It activates plasminogen to form plasmin, which digest fibrin clots. This disrupts the fibrin meshwork which can often form to keep an infection localized.

Staphylokinase interacts with plasminogen and activates other plasminogen molecules. The complex formed by staphylokinase and plasminogen, expose the active site of the plasminogen molecule. That is it does not convert plasminogen to plasmin directly but forms 1:1 stiochemistric complex.The plasmin Sak complex can be actively neutralized by α₂- antiplasmin in plasma in the absence of fibrin. This causes plasminogen activation and consumption to stop and result in lysis. Upon inhibition Sak separates and is used by other plasminogen molecules. Whereas, in presence of fibrin the inhibition is delayed that causes plasminogen activation at the fibrin surface. This is the unique mechanism by which fibrin selectivity in plasma condition is carried out.

Staphylokinase also cleaves IgG and complement component C3b, inhibiting phagocytosis.

It is classified under EC 3.4.24.29, (formerly EC 3.4.99.22).

Structure of Staphylokinase

A full length mature Staphylokinase is 489bp. The first 27 amino acid codes for the signal peptide and from amino acid 28 it starts for full length mature staphyloinase(mSak). The homology was not found between primary structure of Sak and other plasminogen activator. The coding region was followed by Shine-Dalgarno sequence and by -10 and -35 prokaryotic promoter sequence. The natural variants of Sak are Sak42D, SakφC and SakSTAR. These variants had four nucleotide differences in coding region with one silent mutation. The affected codons are amino acid at 38, 61, 63 and 70 in full length sathylokinase. In sathylokinase moieties amino acid 38 is affected is Lys , 61 is Ser SaKSTAR and Gly in SakφC and Arg in Sak42D. Amino acid 63 is Gly in Sak STAR and SakφC but Arg in Sak42D. SakSTAR and SakφC amino acid 70 is His whereas Arg in Sak42D. These reported sequence are suggested as not important protein expression since they are in the 3’ region of staphylokinase molecule and 160 nucleotide downstream of stop codon.

Protein Structure of Staphylokinase

The mature structure of staphylokinase consists of 163 amino acids. In solution the protein structure was analyzed by X-ray scattering, dynamic light scattering, ultracentrifugation and UV circular dichroism spectroscopy. Through these methods the radius of gyration 2.3nm, a stroke radius 2.1nm and dimension maximum of 10nm was obtained which indicated that the shape of staphylokinase is elongated. Sak contains two folded domains which are of similar size. The distance from the center of gravity between two domains is 3.7nm and when in solution the position varies between two domains which state the shape of flexible dumbbell. Staphylokinase was purified by Sodium dodecyl sulphate polyacrylamide gel electrophoresis and iso-electric point and several different molecular forms were obtained after purification. mSak of low molecular weight lacking in 6 and 10 terminal amino acid was observed. mSak on interaction with plasminogen is changed to Sak-Δ10 showing same fibrinolytic activity as mSak. For the activation of plasminogen by staphylokinase amino acid 26 is important.Loss of functionality was observed when amino acid 26 was substituted with Arg or Val.