Trioses, naturally occurring

Some caution should be exercised however when working with synthetic enzymes. A similarly designed and expressed de novo enzyme to mimic the naturally occurring triose phosphate isomerase was reported in 2004 only for the papers to be retracted when it appeared that contamination by unmodified Escherichia coli had been responsible for the observed enzymic activity. For this reason it seems preferential to target reactions that have no known natural catalysts. 

Two main lines of research were developed in Hermann Fischer s Berlin period of 19 -1932. One was the study of the constitution and configuration of the naturally-occurring quinic acid. The other dealt with the difficult chemistry of the trioses, glyceraldehyde and dihydroxyacetone, and related two-, three- and four-carbon compounds. The first publication on quinic acid, which appeared in 1921, described certain new derivatives of this tetrahydroxycyclohexanecarboxyhc acid but left the position of one of the hydroxyl groups uncertain. Four years later. Professor Paul Karrer and his students used this information, and some additional observations, to propose what appeared to be a satisfactory structural formula for the compound. Fischer, however, was not satisfied with this conclusion and... 

The carbohydrates are also divided into two classes, called d and l based on whether they are derived from the trioses D-glyceraldehyde or i-glyceralde-hyde, which have their asymmetric hydroxyl groups to the right and left, respectively. Most of the naturally occurring sugars belong to the D-class. 

Why are these isotopes important in biochemistry and medicine The isotopes we have mentioned occur at very low natural abundance , e.g. in the world around us only about 1 carbon atom in 10 (a million million) is C. However, with the advent of nuclear physics and specifically the Manhattan Project, the atomic bomb project in World War 11, radioactive isotopes started to be produced artificially, and this meant that chemical compounds could be radioactively labelled , either uniformly (e.g. in every carbon position) or selectively (i.e. with radioactive enrichment in particular positions). In the case of carbohydrate metabolism, it was possible to study the relative importance of glycolysis and PPP by comparing the release of radioactivity from glucose, specifically labelled either in carbon 1 or in carbon 6. If you look at Topic 28, you will see that in the initial reactions of the PPP the CO2 that is produced comes entirely from the Cl position. Over time, as the later molecular rearrangements come into play, C6 atoms could also eventually be released but not initially. On the other hand, if you revisit Topics 13 and 14, you will see that, because the sugar phosphate is split down the middle into two triose phosphate halves that are then handled identically, CO2 released in the oxidation of pyruvate to acetyl CoA will be derived equally from Cl and C6. This allows biochemists to assess the relative activities of PPP and glycolysis in different tissues or in the same tissue over time. This is how it was possible to estimate (Topic 28) that 30% of glucose breakdown in liver is via PPP. 

Many of the simple trioses, tetroses, and pentoses do not occur naturally in the free state but are commonly found as phosphate-ester derivatives. The phospho-esters are important intermediates in the breakdown and synthesis of carbohydrates by living organisms. D-Glucose is converted into D-fructose-l,6-bisphos-phate that is then cleaved in half to give D-glyceraldehyde-3-phosphate and dihydroxy acetone phosphate (see Chapter 11). D-Erythrose is found as the 4-phosphate in the pentose-phosphate pathway of carbohydrate degradation and in the photosynthetic process. D-Ribose-5-phosphate, D-ribulose-5-phosphate, D-xy-lose-5-phosphate, and D-xylulose-5-phosphate are found in the pentose phosphate pathway as well as in the photosynthetic pathway (see Chapter 10). D-Ribulose-1,5-bisphosphate is the direct intermediate to which CO2 is added in the photosynthetic pathway. D-Ribose-5-phosphate also is the precursor of RNA (ribonucleic acid) and DNA (deoxyribonucleic acid). See Fig. 1.7 for the structures of these common sugar phosphates. 

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