Cytosol energy’ transport

ENERGY TRANSPORT IN THE CYTOSOL THE CREATINE/PHOSPHOCREATINE SHUHLE... 

Energy transport in the cytosol the creatine/phosphocreatine shuttle... 

The oxidation of one NADH and the reduction of one UQ by NADH-UQ reductase results in the net transport of protons from the matrix side to the cytosolic side of the inner membrane. The cytosolic side, where H accumulates, is referred to as the P (for positive) face similarly, the matrix side is the N (for negative) face. Some of the energy liberated by the flow of electrons... 

ATP results from the movement of approximately three protons from the cytosol into the matrix through Fg. Altogether this means that approximately four protons are transported into the matrix per ATP synthesized. Thus, approximately one-fourth of the energy derived from the respiratory chain (electron transport and oxidative phosphorylation) is expended as the electrochemical energy devoted to mitochondrial ATP-ADP transport. 

Figure 12-14. The creatine phosphate shuttle of heart and skeletal muscle. The shuttle allows rapid transport of high-energy phosphate from the mitochondrial matrix into the cytosol. CKg, creatine kinase concerned with large requirements for ATP, eg, muscular contraction CIC, creatine kinase for maintaining equilibrium between creatine and creatine phosphate and ATP/ADP CKg, creatine kinase coupling glycolysis to creatine phosphate synthesis CK, , mitochondrial creatine kinase mediating creatine phosphate production from ATP formed in oxidative phosphorylation P, pore protein in outer mitochondrial membrane.

Like other cells, a neuron has a nucleus with genetic DNA, although nerve cells cannot divide (replicate) after maturity, and a prominent nucleolus for ribosome synthesis. There are also mitochondria for energy supply as well as a smooth and a rough endoplasmic reticulum for lipid and protein synthesis, and a Golgi apparatus. These are all in a fluid cytosol (cytoplasm), containing enzymes for cell metabolism and NT synthesis and which is surrounded by a phospholipid plasma membrane, impermeable to ions and water-soluble substances. In order to cross the membrane, substances either have to be very lipid soluble or transported by special carrier proteins. It is also the site for NT receptors and the various ion channels important in the control of neuronal excitability. 

Figure 1. Solute transfer across an idealised eukaryote epithelium. The solute must move from the bulk solution (e.g. the external environment, or a body fluid) into an unstirred layer comprising water/mucus secretions, prior to binding to membrane-spanning carrier proteins (and the glycocalyx) which enable solute import. Solutes may then move across the cell by diffusion, or via specific cytosolic carriers, prior to export from the cell. Thus the overall process involves 1. Adsorption 2. Import 3. Solute transfer 4. Export. Some electrolytes may move between the cells (paracellular) by diffusion. The driving force for transport is often an energy-requiring pump (primary transport) located on the basolateral or serosal membrane (blood side), such as an ATPase. Outward electrochemical gradients for other solutes (X+) may drive import of the required solute (M+, metal ion) at the mucosal membrane by an antiporter (AP). Alternatively, the movement of X+ down its electrochemical gradient could enable M+ transport in the same direction across the membrane on a symporter (SP). A, diffusive anion such as chloride. Kl-6 refers to the equilibrium constants for each step in the metal transfer process, Kn indicates that there may be more than one intracellular compartment involved in storage. See the text for details...

In addition to the processes described above, there still remains one further process which, at least in some cells or tissues, is required prior to the utilisation of ATP in the cytosol that is, the transport of energy within the cytosol, via a shuttle. The transport of ATP out and ADP into the mitochondrion, via the translocase, results in a high ATP/ ADP concentration ratio in the cytosol. However, a high ratio means that the actual concentration of ADP in the cytosol is low, which could result in slow diffusion of ADP from a site of ATP utilisation back to the inner mitochondrial membrane. If sufficiently slow, it could limit the rate of ATP generation. To overcome this, a process exists that transports energy within the cytosol, not by diffusion of ATP and ADP, but by the diffusion of phosphocreatine and creatine, a process known as the phosphocreatine/creatine shuttle. The reactions involved in the shuttle in muscle help to explain the significance of the process. They are ... 

Figure 13.15 A diagram representing the Ca ion cycle between the sarcoplasmic reticulum (SR) and the cytosol. The Ca ion release from the SR is down a concentration gradient of about 1000-fold. The re-uptake of Ca ions back into the SR, therefore, reguires ATP hydrolysis to provide the energy to overcome this gradient (i.e. active transport). This is a transport cycle, equivalent to a substrate cycle (Chapter 3). The release of Ca ions is via a Ca ion channel. See text for details of acb vab on of the...

Like the mitosome, most organelles in a eukaryotic cell do not possess a genome and translation machinery. Consequently, proteins must be imported to these compartments from the cytosol. Yet the membranes separating these organelles from the cytosol are not freely permeable for large hydrophilic molecules of proteins. As a result, the protein translocation is mediated by membrane transporters in a complex energy-consuming process. The mode of protein translocation across these barriers is typical for each compartment. 

In animal cells, Na+K+ ATPase maintains the differences in cytosolic and extracellular concentrations of Na+ and K+, and the resulting Na+ gradient is used as the energy source for a variety of secondary active transport processes. 

Although the primary role of the proton gradient in mitochondria is to furnish energy for the synthesis of ATP, the proton-motive force also drives several transport processes essential to oxidative phosphorylation. The inner mitochondrial membrane is generally impermeable to charged species, but two specific systems transport ADP and Pj into the matrix and ATP out to the cytosol (Fig. 19-26). 

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