Triose phosphate isomerase, in converting dihydroxyacetone phosphate into glyceraldehyde 3-phosphate, catalyzes the transfer of a hydrogen atom from C-1 to C-2, that is, catalyzes an intramolecular oxidation-reduction. And in essence, after the enzyme reaction, the carbons C-1, C-2 and C-3 of the starting glucose to become equivalent, chemically indistinguishable, from the carbons C-6, C-5 and C-4, respectively.
Therefore, the net result of the the last two steps of glycolysis is the production of two molecules of glyceraldehyde 3-phosphate .
The ΔG°’ of the reaction is of kJ/mol ( kcal/mol), while the ΔG is kJ/mol ( kcal/mol). Although at equilibrium dihydroxyacetone phosphate represent about 96% of the trioso phosphates, the reaction proceeds readily towards the formation of glyceraldehyde 3-phosphate because of the subsequent step of the glycolytic pathway that removes the glyceraldehyde 3-phosphate produced.
One of the distinguishing features of triose phosphate isomerase is the great catalytic efficiency. The enzyme is in fact considered kinetically perfect . Why? The enzyme enhances the isomerization rate by a factor of 10 10 compared with that obtained with a catalyst such as acetate ion. Indeed, the K cat /K M ratio for the isomerization of glyceraldehyde 3-phosphate is equal to 2×10 8 M -1 s -1 , value close to the diffusion-controlled limit. Thus, the rate-limiting step in the reaction catalyzed by triose phosphate isomerase is diffusion-controlled encounter of enzyme and substrate.
From the energetic point of view, the last two steps of glycolysis are unfavorable, with ΔG°’ of kJ/mol ( kcal/mol), whereas the net ΔG°’ of the first five reactions is of kJ/mol ( kcal/mol), with a K eq of about . And it is the free energy derived from the hydrolysis two ATP that, under standard-state conditions, makes the value of the overall equilibrium constant close to one. If instead we consider ΔG, it is quite negative, - kJ/mol (- kcal/mol).
Testosterone can be administered parenterally , but it has more irregular prolonged absorption time and greater activity in muscle in enanthate , undecanoate , or cypionate ester form. These derivatives are hydrolyzed to release free testosterone at the site of injection; absorption rate (and thus injection schedule) varies among different esters, but medical injections are normally done anywhere between semi-weekly to once every 12 weeks. A more frequent schedule may be desirable in order to maintain a more constant level of hormone in the system.  Injectable steroids are typically administered into the muscle, not into the vein, to avoid sudden changes in the amount of the drug in the bloodstream. In addition, because estered testosterone is dissolved in oil, intravenous injection has the potential to cause a dangerous embolism (clot) in the bloodstream.
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