Pathways

The L-arabinose operon, also called the ara or araBAD operon, is an operon required for the breakdown of the five-carbon sugar, L-arabinose, in Escherichia coli. The L-arabinose operon contains three structural genes: araB, araA, araD (collectively known as araBAD), which encode for three metabolic enzymes that are required for the metabolism of L-arabinose. AraB (ribulokinase), AraA (an isomerase), AraD (an epimerase) produced by these genes catalyse conversion of L-arabinose to an intermediate of the pentose phosphate pathway, D-xylulose-5-phosphate. The structural genes of the L-arabinose operon are transcribed from a common promoter into a single transcript, a mRNA. The expression of the L-arabinose operon is controlled as a single unit by the product of regulatory gene araC and the catabolite activator protein (CAP)-cAMP complex. The regulator protein AraC is sensitive to the level of arabinose and plays a dual role as both an activator in the presence of arabinose and a repressor in the absence of arabinose to regulate the expression of araBAD. AraC protein not only controls the expression of araBAD but also auto-regulates its own expression at high AraC levels. ----------------------------------- Negative regulation of araBAD Negative regulation of L-arabinose operon via AraC protein: When arabinose is absent, cells do not need the araBAD products for breaking down arabinose. Therefore, dimeric AraC acts as a repressor: one monomer binds to the operator of the araBAD gene (araO2), another monomer binds to a distant DNA half site known as araI1. This leads to the formation of a DNA loop. This orientation blocks RNA polymerase from binding to the araBAD promoter. Therefore, transcription of structural gene araBAD is inhibited. ----------------------------------- Positive regulation of araBAD Positive regulation of L-arabinose operon via dimeric AraC and CAP/cAMP: Expression of the araBAD operon is activated in the absence of glucose and in the presence of arabinose. When arabinose is present, both AraC and CAP work together and function as activators. ----------------------------------- Autoregulation of AraC: The expression of araC is negatively regulated by its own protein product, AraC. The excess AraC binds to the operator of the araC gene, araO1, at high AraC levels, which physically blocks the RNA polymerase from accessing the araC promoter. Therefore, the AraC protein inhibits its own expression at high concentrations.

The L-arabinose operon, also called the ara or araBAD operon, is an operon required for the breakdown of the five-carbon sugar, L-arabinose, in Escherichia coli. The L-arabinose operon contains three structural genes: araB, araA, araD (collectively known as araBAD), which encode for three metabolic enzymes that are required for the metabolism of L-arabinose. AraB (ribulokinase), AraA (an isomerase), AraD (an epimerase) produced by these genes catalyse conversion of L-arabinose to an intermediate of the pentose phosphate pathway, D-xylulose-5-phosphate. The structural genes of the L-arabinose operon are transcribed from a common promoter into a single transcript, a mRNA. The expression of the L-arabinose operon is controlled as a single unit by the product of regulatory gene araC and the catabolite activator protein (CAP)-cAMP complex. The regulator protein AraC is sensitive to the level of arabinose and plays a dual role as both an activator in the presence of arabinose and a repressor in the absence of arabinose to regulate the expression of araBAD. AraC protein not only controls the expression of araBAD but also auto-regulates its own expression at high AraC levels. ----------------------------------- Negative regulation of araBAD Negative regulation of L-arabinose operon via AraC protein: When arabinose is absent, cells do not need the araBAD products for breaking down arabinose. Therefore, dimeric AraC acts as a repressor: one monomer binds to the operator of the araBAD gene (araO2), another monomer binds to a distant DNA half site known as araI1. This leads to the formation of a DNA loop. This orientation blocks RNA polymerase from binding to the araBAD promoter. Therefore, transcription of structural gene araBAD is inhibited. ----------------------------------- Positive regulation of araBAD Positive regulation of L-arabinose operon via dimeric AraC and CAP/cAMP: Expression of the araBAD operon is activated in the absence of glucose and in the presence of arabinose. When arabinose is present, both AraC and CAP work together and function as activators. ----------------------------------- Autoregulation of AraC: The expression of araC is negatively regulated by its own protein product, AraC. The excess AraC binds to the operator of the araC gene, araO1, at high AraC levels, which physically blocks the RNA polymerase from accessing the araC promoter. Therefore, the AraC protein inhibits its own expression at high concentrations.

The L-arabinose operon, also called the ara or araBAD operon, is an operon required for the breakdown of the five-carbon sugar, L-arabinose, in Escherichia coli. The L-arabinose operon contains three structural genes: araB, araA, araD (collectively known as araBAD), which encode for three metabolic enzymes that are required for the metabolism of L-arabinose. AraB (ribulokinase), AraA (an isomerase), AraD (an epimerase) produced by these genes catalyse conversion of L-arabinose to an intermediate of the pentose phosphate pathway, D-xylulose-5-phosphate. The structural genes of the L-arabinose operon are transcribed from a common promoter into a single transcript, a mRNA. The expression of the L-arabinose operon is controlled as a single unit by the product of regulatory gene araC and the catabolite activator protein (CAP)-cAMP complex. The regulator protein AraC is sensitive to the level of arabinose and plays a dual role as both an activator in the presence of arabinose and a repressor in the absence of arabinose to regulate the expression of araBAD. AraC protein not only controls the expression of araBAD but also auto-regulates its own expression at high AraC levels. ----------------------------------- Negative regulation of araBAD Negative regulation of L-arabinose operon via AraC protein: When arabinose is absent, cells do not need the araBAD products for breaking down arabinose. Therefore, dimeric AraC acts as a repressor: one monomer binds to the operator of the araBAD gene (araO2), another monomer binds to a distant DNA half site known as araI1. This leads to the formation of a DNA loop. This orientation blocks RNA polymerase from binding to the araBAD promoter. Therefore, transcription of structural gene araBAD is inhibited. ----------------------------------- Positive regulation of araBAD Positive regulation of L-arabinose operon via dimeric AraC and CAP/cAMP: Expression of the araBAD operon is activated in the absence of glucose and in the presence of arabinose. When arabinose is present, both AraC and CAP work together and function as activators. ----------------------------------- Autoregulation of AraC: The expression of araC is negatively regulated by its own protein product, AraC. The excess AraC binds to the operator of the araC gene, araO1, at high AraC levels, which physically blocks the RNA polymerase from accessing the araC promoter. Therefore, the AraC protein inhibits its own expression at high concentrations.

The araBAD operon in E. coli contains three genes which encode proteins involved in the metabolism of L-arabinose into a form useable by the pentose phosphate pathway. The operon can be repressed by the arabinose operon regulatory protein, a protein dimer that is produced in the cell. When arabinose is not present, it binds to the DNA in two specific locations, forming a loop in the DNA and preventing transcription from occurring. Inversely, the operon can be activated by the same arabinose operon regulatory protein in the presence of arabinose. In this case, the arabinose binds to the protein dimer, preventing it from forming the DNA loop and instead binding to the activator binding site upstream of the promoter. This binding helps RNA polymerase to bind and transcribe the operon. Finally, the araBAD operon functions like the lac operon, which only produces its enzymes in the absence of glucose. This is regulated by the cAMP-activated global transcriptional regulator (CRP). In the absence of glucose, cAMP levels build up in the cell, and it can bind to and activate CRP. The presence of both activated CRP and the arabinose operon regulatory protein promote the binding of RNA polymerase. The first gene in the operon, araB, encodes ribulokinase, an enzyme that irreversibly converts L-ribulose to L-ribulose-5-phsophate. The second gene, araA, encodes L-arabinose isomerase, an enzyme that convertsz L-arabinose to L-ribulose. This enzyme is the first step in the metabolism of arabinose. In between araA and araD are several non-coding extragenic sites. Finally, the last gene in the operon, araD, encodes L-ribulose-5-phosphate 4-epimerase, a protein that converts L-ribulose-5-phosphate to D-xylulose-5-phosphate, which then is used in the pentose phosphate pathway.

Aprotinin is an inhibitor that is used to prevent blood loss during cardiopulmonary bypass surgery. It slows the breakdown of fibrin clots to reduce bleeding by inhibiting the proteins plasminogen and kallikrein. The drug is slowly metabolized by lysosomal enzymes and is eliminated through the urine over 48 hours. The drug was withdrawn worldwide due to the enhanced risks associated with its use leading to various complications and in some cases death.

Aprotinin is an inhibitor that is used to prevent blood loss during cardiopulmonary bypass surgery. It slows the breakdown of fibrin clots to reduce bleeding by inhibiting the proteins plasminogen and kallikrein. The drug is slowly metabolized by lysosomal enzymes and is eliminated through the urine over 48 hours. The drug was withdrawn worldwide due to the enhanced risks associated with its use leading to various complications and in some cases death.

Aprotinin is a naturally occurring serine protease inhibitor derived from bovine lung. It is administered intravenously as an antifibrinolytic agent for prophylactic use to reduce perioperative blood loss and the need for blood transfusion in patients undergoing cardiopulmonary bypass in the course of coronary artery bypass graft surgery who are at an increased risk for blood loss and blood transfusion. The clotting process consists of two pathways, intrinsic and extrinsic, which converge to create stable fibrin which traps platelets and forms a hemostatic plug. The intrinsic pathway is activated by trauma inside the vasculature system, when there is exposed endothelial collagen. Endothelial collagen only becomes exposed when there is damage. The pathway starts with plasma kallikrein activating factor XII. The activated factor XIIa activates factor XI. Factor IX is then activated by factor XIa. Thrombin activates factor VIII and a Calicum-phospholipid-XIIa-VIIIa complex forms. This complex then activates factor X, the merging point of the two pathways. The extrinsic pathway is activated when external trauma causes blood to escape the vasculature system. Activation occurs through tissue factor released by endothelial cells after external damage. The tissue factor is a cellular receptor for factor VII. In the presence of calcium, the active site transitions and a TF-VIIa complex is formed. This complex aids in activation of factors IX and X. Factor V is activated by thrombin in the presence of calcium, then the activated factor Xa, in the presence of phospholipid, calcium and factor Va can convert prothrombin to thrombin. The extrinsic pathway occurs first, producing a small amount of thrombin, which then acts as a positive feedback on several components to increase the thrombin production. Thrombin converts fibrinogen to a loose, unstable fibrin and also activates factor XIII. Factors XIIIa strengthens the fibrin-fibrin and forms a stable, mesh fibrin which is essential for clot formation. The blood clot can be broken down by the enzyme plasmin. Plasmin is formed from plasminogen by tissue plasminogen activator. Aprotinin inhibits plasmin, preventing the degradation of the fibrin clot and reducing bleeding. Aprotinin also inhibits plasma kallikrein. This prevents the formation of factor XIIa, thereby inhibiting the intrinsic pathway of the coagulation cascade and reducing coagulation. When blood is subjected to extracorporeal circulation during cardiopulmonary bypass surgery, there are certain changes in normal coagulation and clotting properties. Clotting mechanisms that are activated through surface-media contact increases thrombotic and fibrinolytic activity and is responsible for excessive bleeding. This is why aprotinin targets both coagulation and fibrinolysis, to re-establish normal homeostatic balance of clot formation and clot lysis. Side effects of aprotinin include atrial fibrillation, fever, nausea, low blood pressure, lung problems, kidney disease, blood clots, heart failure, severe allergic reaction (anaphylaxis), stroke, or trouble breathing.

Aprotinin is an inhibitor that is used to prevent blood loss during cardiopulmonary bypass surgery. It slows the breakdown of fibrin clots to reduce bleeding plasminogen and kallikrein. The drug is slowly metabolized by lysosomal enzymes and is eliminated through the urine over 48 hours.

Aprotinin, trade name Trasylol is a bovine serine protease inhibitor of trypsin, chymotrypsin, kallikrein and plasmin. Aprotinin is administered prophylactically to patients undergoing surgery with a high risk of bleeding to block fibrinolysis and prevent the breakdown of blood clots. The inhibition of kallikrein inhibits factor XIIa production to reduce fibrinolysis and coagulation. Inhibition of plasmin slows down fibrinolysis. Hypersensitivity reactions may occur after repeat exposure to Aprotinin.