Hydrophobicity fibrinogen

Alpha helices are sufficiently versatile to produce many very different classes of structures. In membrane-bound proteins, the regions inside the membranes are frequently a helices whose surfaces are covered by hydrophobic side chains suitable for the hydrophobic environment inside the membranes. Membrane-bound proteins are described in Chapter 12. Alpha helices are also frequently used to produce structural and motile proteins with various different properties and functions. These can be typical fibrous proteins such as keratin, which is present in skin, hair, and feathers, or parts of the cellular machinery such as fibrinogen or the muscle proteins myosin and dystrophin. These a-helical proteins will be discussed in Chapter 14. 

Hydrophobically modified HA derivatives,91 obtained through the partial esterification of the HA carboxyl groups with methylprednisolone (45% in HYCp45 and 60% in HYCp60),92 have been deeply studied 93 A key point prior to any in vivo study of the biomaterial is the assessment of the so-called "stealth character" of the species itself. Such characteristic corresponds to be invisible towards the immune system, so that colloids are not recognized as foreign objects by body fluid components, as plasma proteins fibrinogen, BSA and lipidic components.94,95... 

The most widely studied synthetic polymers for blood contact applications are polyether urethane ureas ( Biomer (Ethicon)). These materials have been used in artificial hearts, as coatings for lead wires in pacemakers, have been used and are being considered for blood vessel prostheses. The success of these materials is believed to be due to preferential adsorption of albumin rather than globulin or fibrinogen which promote a clotting response. However, these materials are hydrophobic and questions of long-term effectiveness are unresolved. Particularly, these materials may shed emboli or may be susceptible to surface calcification. Thus, it may be desirable to have synthetic polymers which are hydrophilic and better resemble blood vessels [475]. 

The combination of the hydrophobic salting out and the electrostatic salting in terms explains very nicely the Class I type behavior. The data for carboxyhemoglobin and fibrinogen are well fitted by the theory (Figure 10). Also the order of decreasing molal surface tension increment generally follows the lyotropic order (7). 

Studies of the role of protein-surface interactions in blood coagulation were done by Vroman 56). The plasma proteins were adsorbed onto various hydrophilic or hydrophobic surfaces. Vroman showed that fibrinogen was an important component of the plasma protein layer adsorbed to the solid/liquid interface. 

Fig. 33.1. Structure of DNA aptamers for recognition fibrinogen (A) [15,29] and heparin binding sites (B) [17] of thrombin. The non-canonical hydrogen bonds between guanines are shown by dotted lines. In the case of structure (A), the spacer composed of 15 T chain terminated by thiol group at the end of hydrophobic spacer is shown [34]. (C) Structure of RNA aptamer against fibrinogen binding site of thrombin [23].

Wertz CF, Santore MM (1999) Adsorption and relaxation kinetics of albumin and fibrinogen on hydrophobic surfaces single-species and competitive behavior. Langmuir 15(26) 8884-8894... 

Adsorption of albumin, y-globulin, and fibrinogen from single solutions onto several hydrophobic polymers was studied using internal reflection IR spectroscopy. The adsorption isotherms have a Langmuir-type form. The calculated rate and amount of protein adsorbed was dependent on the polymer substrate and the flow rate of the solution. Competitive adsorption experiments were also investigated to determine the specific adsorption of each I-labelled protein from a mixture of proteins. Platelet adhesion to these proteinated surfaces is discussed in relation to a model previously proposed. 

Studies of adsorption of albumin, y-globulin, and fibrinogen onto several polymer membranes including cation exchangers have been carried out by Dillman and Miller 15), and two types of adsorption were characterized. One is relatively hydrophilic, exothermic, and easily reversible with a heat of adsorption of ca. —10 kcal/mole. The other is apparently tightly bound, hydrophobic, and endothermic with heats of adsorption ranging from 5 to 20 kcal/mole. 

Our studies (19) indicated that proteins were readily adsorbed from aqueous solution onto hydrophobic polymer surfaces with Langmuir type adsorption and that the rate of adsorption toward a plateau surface concentration depends on the polymer nature. In the study of competitive adsorption from a protein mixture solution (20), fibrinogen and y-globulin adsorb onto FEP very rapidly compared with PEUU and SR. Therefore, the FEP surface in contact with blood has more acceptor sites for platelet adhesion than does the PEUU or SR surface. 

Platelets at air interfaces may adhere to a film of protein formed by the plasma. This film appears to contain fibrinogen in a state that will not allow the plasma to convert it (114) thus, more platelets may adhere to this unconvertable fibrinogen than elsewhere. On a hydrophobic solid, an advancing drop of blood would transfer both the protein film and the platelets on it hence the great loss of platelets found at hydrophobic surfaces in presence of an air interface (101). 

Tanaka, A. and Fujiwara, H. (1996). Quantitative Structure-Activity Relationship Study of Fibrinogen Inhibitors, ((4-(4-Amidinophenoxy)Butanoyl)Aspartyl)Valine (FK633) Derivatives, Using a Novel Hydrophobic Descriptor. J.MetLChem., 39,5017-5020. 

Other investigations have been performed where silicon substrates were modified from alkaline to acidic and hydrophobic nature [58], where salt concentration was varied during adsorption [55] and where the nature of one of the blocks was changed [59]. Monolayers of polyampholyte micelles were used to modify protein adsorption [60]. An example is shown in Fig. 18 where the adsorption of fibrinogen is controlled by preadsorption of polyampholyte micelles and the phosphate buffer concentration. 

Plasmin is a protease with specificity for arginine or lysine as amino acids participating in bond cleavage.65 Examination of the preferred sites for plasmin cleavage of the fibrinogen molecule (the physiological function of plasmin) shows that the preferred sites involve lysine linked to a hydrophobic amino acid. Thus, the choice of the peptidic sequence to be used has focused on a peptide having a hydro-phobic amino acid linked to lysine.66... 

X-ray analysis of crystals of iohexol and serine proteinase (pancreaticporcine elastase) reveals that three molecules of iohexol are associated with elastase, with one close to the active site (subsite SI), the second in the vicinity of in subsites S2/S3, and the third located in a pocket at the surface of the protein. The association is a result of the affinity of iohexol directed toward the hydrophobic regions of the enzyme and supports the hypothesis of the contrast medium s potent inhibition of thrombin. Another example of the contrast medium-protein interaction is between iopamidol and fibrinogen or lysozyme... 

In thrombin, the specificity extends far beyond the residue preceding the cleavage site. NMR [4] and X-ray studies [3, 5] revealed that three hydrophobic residues in fibrinogen 2 two, eight and nine residues before the cleavage site, respectively, occupy unique hydrophobic pockets (Fig. 3). These hydrophobic pockets are formed by a unique additional loop Tyr-Pro-Pro-Trp above the active site. 

Tripeptides of the type X-Pro-Y-Z that are derived from the NH2 terminal sequence of the fibrinogen a-chain have been widely investigated as active-site directed thrombin inhibitors [68]. For maximum affinity X requires a hydrophobic residue in the if configuration and Z is a non hydrolyzable function. An arginyl residue is optimal for Y... 

Tanaka, A. and Fujiwara, FI. (1996) Quantitative structure-activity relationship study of fibrinogen inhibitors ((4-(4-amidinophenoxy)butanoyl) aspartyljvaline (FK633) derivatives, using a novel hydrophobic descriptor. /. Med. Chem.,... 

Imagine for a moment what would happen if the pH of the blood were to become too acidic or too basic. Blood is a fluid that contains water and dissolved electrolytes, a variety of cells, including the red blood cells responsible for oxygen transport, and many different proteins. These proteins include fibrinogen, which is involved in the clotting reaction immunoglobulins, which protect us from disease and albumins, which carry hydrophobic molecules in the blood. 

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