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All cells contain the enzyme hexokinase, which catalyzes the conversion of glucose that has entered the cell into glucose-6-phosphate (G6P). Since the cell membrane is impervious to G6P, hexokinase essentially acts to transport glucose into the cells from which it can then no longer escape. Hexokinase is inhibited by high levels of G6P in the cell.
Thermodynamically the flow of substances from one compartment to another can occur in the direction of a concentration or electrochemical gradient or against it. If the exchange of substances occurs in the direction of the gradient, that is, in the direction of decreasing potential, there is no requirement for an input of energy from outside the system; if, however, the transport is against ...
The G beta-gamma complex (G βγ) is a tightly bound dimeric protein complex, composed of one G β and one G γ subunit, and is a component of heterotrimeric G proteins. Heterotrimeric G proteins, also called guanine nucleotide-binding proteins, consist of three subunits, called alpha , beta, and gamma subunits , or G α , G β , and G γ .
This form is composed of four identical 37-kDa subunits containing a single catalytic thiol group each and critical to the enzyme's catalytic function. [6] [7] Nuclear GAPDH has increased isoelectric point (pI) of pH 8.3–8.7. [7] Of note, the cysteine residue C152 in the enzyme's active site is required for the induction of apoptosis by ...
[9] [10] Liver cells are freely permeable to glucose, and the initial rate of phosphorylation of glucose is the rate-limiting step in glucose metabolism by the liver. [ 9 ] The liver's crucial role in controlling blood sugar concentrations by breaking down glucose into carbon dioxide and glycogen is characterized by the negative Gibbs free ...
Upon reaching the plasmalemma, the vesicles fuse with the membrane, increasing the number of GLUT4 transporters expressed at the cell surface, and hence increasing glucose uptake. GLUT4 has a Km value for glucose of about 5 mM, which as stated above is the normal blood glucose level in healthy individuals.
Genes coding for the enzyme are primarily expressed in the liver, in the kidney cortex and (to a lesser extent) in the β-cells of the pancreatic islets and intestinal mucosa (especially during times of starvation). [7] Glucose 6-phosphatase is present in a wide variety of muscles across the animal kingdom, albeit at very low concentrations. [10]
G protein alpha subunits bind to guanine nucleotides and function in a regulatory cycle, and are active when bound to GTP but inactive and associated with the G beta-gamma complex when bound to GDP. [3] [4] G 12 /G 13 are not targets of pertussis toxin or cholera toxin, as are other classes of G protein alpha subunits. [5]