Fixation mechanism

The question is therefore, what are the principal requirements of an autotrophic carbon-fixation mechanism An organic molecule serves as a C02 acceptor molecule, which becomes carboxylated by a carboxylase enzyme. This C02 acceptor molecule needs to be regenerated in a reductive autocatalytic cycle. The product that can be drained off from such a metabolic cycle should be a central cellular metabolite, from which all cellular building blocks for polymers can be derived examples of such central metabolites are acetyl-CoA, pyruvate, oxaloacetate, 2-oxoghitarate, phosphoe-nolpyruvate, and 3-phosphoglycerate. Importantly, the intermediates should not be toxic to the cell. The irreversible steps of the pathway are driven by ATP hydrolysis, while the reduction steps are driven by low-potential reduced coenzymes. 

Recently we have attempted to pursue multi-electron fixation processes as models for N2 or CO2 fixation. In nature, the N2-fixation enzyme, nitrogenase, exhibits non-specificity properties, and acetylene competes for nitrogen as the fixation substrate (21). The fixation process of acetylene to methane and of nitrogen to ammonia (euqations 14 and 15) have several common features (i) both involve the cleavage of a triple bond (ii) the two reactions involve 6 electrons in the fixation mechanism. Thus, it seems that the photocleavage of acetylene to methane might offer a good model for development of -fixation cycles (22). 

Macrodistribution. The ability of any wood preservative to control biodegradation is affected by the macrodistribution of the chemical within the wood product being protected. The macrodistribution of a preservative is influenced by three basic factors wood characteristics, treating process, and characteristics of the treating solution. Consideration of the principles of flow in wood and of the factors that influence the treatment of wood are covered in Chapters 3 and 4 14, 15). Suffice it to say that when the preservative has been distributed through the wood, fixation will occur either through chemical interaction between the preservative and the wood structure, between the preservative components themselves, or by physical deposition as a result of solvent loss. These fixation mechanisms are covered in the section on microdistribution. 

The lignin hypothesis implies that the active copper fungitoxicant is fixed by ion exchange to the lignin. The importance of the fixation mechanisms has not been accounted for, but soft-rot attack in hardwood treated with CCA was accelerated when the wood was subjected to rapid fixation of the preservative at elevated temperatures after treatment (3J). Undoubtedly, the change in fixation mechanisms caused by the heat treatment has a marked effect on the control of soft-rot decay. This observation is also compatible with the fixation and lignin hypothesis theories. 

Inorganic Salt Preservatives. Inorganic compounds used to formulate wood preservatives are normally water soluble. As a result, the salts deposited in the wood are susceptible to leaching unless they are transformed to insoluble compounds or are chemically fixed to the wood substrate. Indeed, such conversions do occur as a result of interactions between some of the salts and the wood substrate. Both single element and multicomponent reactions are involved in the fixation mechanisms copper and chromium are the most reactive of the possible components. 

C02-fixation mechanism is referred to as CAM-(crassulacean acid metabolism) typ2S ... 

Soballe, K. (1993) Hydroxyapatite ceramic coating for bone implant fixation. Mechanical and histological studies in dogs. Acta Orthop. Scand., 64 (Suppl. 255), 58 pp. 

Each of the fixation mechanisms has an idiosyncratic behavior, and their load transfer characteristics as well as the failure mechanisms are different. Further complexity arises from prostheses which combine two or more of the fixation mechanisms in different regions of the implant. Multiple mechanisms of fixation are used in an effort to customize load transfer to requirements of different regions of bone in an effort to preserve bone mass. Loosening, unlocking, or de-bonding between implant and bone constitute some of the most important mechanisms of prosthetic failure. 

In addition, cyanobacteria possess a second carbon fixation mechanism through phosphoenolpyruvate carboxylase (PPC) that is responsible for close to 25% of CO2 fixation [102]. PPC fixes HCOg" rather than CO2 and combines it with phosphoenolpyruvate to form oxaloacetate and inorganic phosphate. An in silico modeling study has proposed to couple PPC with the core C4 plant carbon fixation cycle to Increase the overall carbon fixation rate compare to the CBB cycle [103]. 

Obviously, fixation mechanisms and electrode-tissue interaction have crucial implications for lead extraction also. Encasement of the tines of a passive fixation lead by fibrous reaction may make transvenous lead removal more difficult than that of an active-fixation isodiametric leads. 

As with pacing leads, the ICD lead cathode is a platinum, iridium, and titanium combination the electrode structure depends on whether fixation mechanism is passive with tines or active with a screw. In an active-fixation lead, the platinum-iridium screw is usually electrically active, but there are also models with both electrically active collar and helix or others with electrically silent helix and active carbon cathode collar. A platinum-iridium anode lies immediately behind the cathode in a true bipolar system. All models now elute steroids to reduce chronic threshold and prevent pacing threshold rise or exit block. 

Modern ICD lead body design is multiluminal separated lumens contain, in parallel, single cable or coil conductors according to their role. The tip (cathode) electrode conductor is coiled to allow insertion of a stylet and attachment of a screw fixation mechanism, whereas anode-electrode and defibrillation coil conductors are cabled. All conductors are coated with a Teflon-like fluoropolymer... 

Fixation mechanism Active fixation leads seem to have advantages and disadvantages. On the one hand, once the screw is retracted, the tip can be easily detached from the endocardium, and the lead, being isodiametric, can be removed more easily than the others [6, 7]. On the other hand, an unscrewed lead is more difficult than a tined one. Fixation mechanisms that cannot be unscrewed, such as the Vitatron Helifix, make the procedure more complex, increasing the risks of myocardial perforation and cardiac tamponade. 

Before starting the extraction procedure, the operator must identify the type and manufacturer of leads submitted to removal. One must identify lead fixation mechanism and type of fixation in case of an active fixation lead in some active fixation leads, special stylets are needed in order to unscrew the screw. After lead identification, the procedure of lead extraction consists of lead preparation and sheath application when gentle manual traction is unsuccessful. 

The shorter implant time, different characteristics of lead insulation, and absence of a fixation mechanism can explain the substantial ease of transvenous removal of LV leads. Regardless of the ease of lead removal by simple traction, binding sites into the CS are present in most patients as evidenced by findings during LV lead reimplantation. These include CS stenosis, presence of thrombi, and obliteration of venous branches. These observations suggest that, due to their different characteristics, it may be easier to slip LV leads through adhesions than the other leads. On the other hand, these observations and clinical experiences suggest that the use of fixation mech-... 

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