Publisher Summary This chapter focuses on protein–protein interaction. The unique characteristics of different proteins are developed in the interactions of proteins with each other, with ions, and with nonprotein materials such as lipids, nucleic acids, and carbohydrates. These range from covalent bonds and electrostatic interactions to the weaker electronic van der Waals' attractive forces characteristic of all atoms and groups of atoms. The interactions of proteins with each other in an aqueous environment may be divided into two categories: (1) those which take place at such distances that the average properties of the protein or its surface may be used in calculating interaction energies, and (2) those in which the interacting molecules approach closely and thus bring into play local forces, the total of which depends to a major extension specific groups and group configurations. Interactions at distances may be further subdivided into those interactions, which lead to a concentration dependence of osmotic pressure, sedimentation coefficient, and those long-range interactions which lead to more complex phenomena such as the separation of phases in suspensions of tobacco mosaic virus and bentonite. An interaction is observed only when the protein molecule is subject to a structural change which brings bonding groups into positions which allow them to interact. Structural changes of this type may be reversible or irreversible. Certain ephemeral interactions, particularly enzymatic reactions, generally lead to chemical or physical changes in one of the interactants to a much greater extent than the other.

Protein-protein interactions.

Publisher Summary The thrombin-induced conversion of fibrinogen to fibrin that is the primary subject of this review is essentially the final stage in the complex series of processes involved in blood coagulation. Fibrinogen exists in normal blood, while thrombin is present as its precursor prothrombin. The coagulation process consists of the conversion of prothrombin to thrombin and of the subsequent action of thrombin on fibrinogen to produce fibrin. In the fibrinogen-fibrin conversion fibrinogen, already a high polymer of many amino acids is converted to an even higher polymeric form or coagulum, the fibrin clot. Prothrombin can be converted to thrombin in a series of highly complex, and as yet not well understood, reactions. The chapter explores four of the processes by which the conversion of prothrombin to thrombin takes place. These are (1) activation by citrate, (2) activation by calcium ions, Ac-globulin and thromboplastin, (3) activation by calcium ions, platelets and thromboplastin, and (4) activation by trypsin. The chapter concludes by addressing the various problems of current interest in protein chemistry, which appear in the study of the fibrinogen-fibrin conversion. The first step is an example of a limited proteolysis by a protease of curious specificity. This proteolysis may turn out to be one of the first illustrations of the existence of “strong” peptide bonds in proteins. Further, it offers a nice illustration of the phenomenon of unmasking or activation in biological systems. The second step involves protein-protein associations with unequal association constants. The last step may involve gel formation or crystallization.

The Fibrinogen-Fibrin Conversion1

A glance at FIGURE l a reveals the built-in regulatory features of the physiological process in vertebrates. I t may be noted that the scheme was derived entirely from a study of the individual reaction steps in isolated systems. A complete description would require insertion of additional intermediates, and FIGURE l a is to be taken only as a minimal representation without which (as we shall see later) correct differential diagnosis of disorders in this field simply would not be possible. Thrombin was shown to act as a proteolytic enzyme both l-* on fibrinogen and on fibrin-stabilizing factor (Factor XIII) .t It coordinates the appearance of fibrin with the formation of fibrinoligase from its zymogen. As such, the physiologically important sequence of clotting followed by enzymatic crosslinking is ensured. Clotting in lobster plasma (FIGURE lb ) does not seem to depend on hydrolysis by a thrombin-like enzyme, and the process can be conceived of as a rudimentary shunt of the more complicated system typical for vertebrates. In Homarus, an intracellular transamidase (transglutaminase) functions as the clotting enzyme.\"-\" It is released during clumping of amoebocytes or during damage to muscle tissue. Intracellular compartmentalization of the already active enzyme is apparently the only means of control for preserving the fluidity of lobster plasma. Small synthetic amines which inhibit fibrinol i g a ~ e , ~ also inhibit the lobster enzymeeR These compounds affect only the cross-linking step in vertebrates and do not interfere with the appearance of a clot. When added to lobster plasma, they delay the onset of clotting itself. Although fibrinoligase and the lobster enzyme both belong to a group of enzymes which catalyze the calcium-dependent incorporation of amines into proteins (e.g., putrescine into casein), their specificities for the natural substrates are so different that they cannot be readily interchanged. In a phenomenological

Fibrinoligase: the fibrin‐stabilizing factor system of blood plasma*

Fibrin gel structure and clotting time.

So each man strives to flee that secret foe Which is himself. But move he swift or slow, That self, for ever punctual at his heels, Never for one short hour will let him go.

Fibrinogen to Fibrin Transformation

Distribution of intermediate polymers in the fibrinogen-fibrin conversion.

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Distribution of intermediate polymers in the fibrinogen-fibrin conversion.