Ribulose degradation

In the pentose plu phatc pathway for degrading sugars, ribulose 5-phosphate is converted to ribose 5-phosphale. Propose a mechanism for the isomerization. 

The oxidative segment of the PPP converts glucose 6-phosphate to ribulose 5-phosphate. One CO2 and two NADPH+H" are formed in the process. Depending on the metabolic state, the much more complex regenerative part of the pathway (see B) can convert some of the pentose phosphates back to hexose phosphates, or it can pass them on to glycolysis for breakdown. In most cells, less than 10% of glucose 6-phosphate is degraded via the pentose phosphate pathway. 

In tha pentode pko phate pathway for degrading sugara, ribulose 5- phos.p(iate is converted to ribose Q-phosphate. Propo e a mechanism for the it merization. 

Are the proteins identified in the previous section selectively preserved in DOM In general, porins are expected to be resistant to bacterial degradation (Powell et al. (2005) and reference therein), and modification of these proteins, by saccharide moieties for example, may further retard their bacterial degradation. For example, in a laboratory experiment Keil and Kirchman (1993) noted longer turnover times for glycosylated ribulose 1,5-bisphosphate carboxylase (RuBPCase) when compared with unmodified RuBPCase. In accordance with this report Suzuki et al. (1997) suggested that the glycosylation of porins could help to explain their enhanced preservation/accumulation in seawater. Subsequently Yamada and Tanoue (2003)... 

Methylation. Protein methylation serves several purposes in eukaryotes. The methylation of altered aspartate residues by a specific type of methyltransferase promotes either the repair or the degradation of damaged proteins. Other methyl-transferases catalyze reactions that alter the cellular roles of certain proteins. For example, methylated lysine residues have been found in such disparate proteins as ribulose-2,3-bisphosphate carboxylase, calmodulin, histones, certain ribo-... 

After a time, several other sugar phosphates were identified. Most important among these were the seven-carbon compounds, sedoheptulose-7-phosphate (IX) and sedoheptulose-l,7-diphosphate (SDP) (X), and the five-carbon compounds, ribulose-l,5-diphos-phate (RuDP) (II) and ribose-5-phosphate (XI), xylulose-5-phosphate (XII), and ribulose-5-phosphate (I) (Benson, et al., 1952). The roles of these compounds in the path of carbon in photosynthesis became more clear after they had been degraded to locate the position of radiocarbon atoms within the individual molecules (Bassham et al., 1954). 

The end result of Eq. (13)-(22) is the conversion of five molecules of glyceraldehyde-3-phosphate to three molecules of ribulose-5-phosphate (see Fig. 6). Two of these molecules formed by Eq. (21) and (22) are labeled in carbon atom position 3, while the third one, from Eq. (19) and (20), is labeled in positions 1, 2, and 3. The resultant average labeling of ribulose phosphate is heavy in position 3 and lighter in positions 1 and 2. When the ribulose molecules, labeled after a few seconds photosynthesis with C 02, were degraded (Bassham et al., 1954), this pattern of... 

Methods for degrading ribulose (n-er / Aro-pentulose) and sedoheptulose (n-aZ ro-heptulose) have been devised by Bassham and associates 46). Biological methods of degradation depending on the action of bacteria to produce small fragments such as ethanol, acetic acid, formic acid, lactic acid, etc., are of rather wide use in the sugar field. 

Fig. 8.3 The degradation of the large subunit (LSU) and small subunit (SSU) of ribulose-l,5-bis-phosphate carboxylase/oxygenase (Ruhisco) during in vitro incubation of toted ieeif protein extracted from white clover (Trifolium repens) with Streptococcus bovis rumen bacteried strrdns (105 cells innoculated) and condensed tannins extracted from Lotus comicidatus (birdsfoot trefoil). Incubations were performed with (b, CT-active) and without CT (a, CT-inactive). Sampies were removed prior (0 h) and after 2, 4, 8, 12, and 24 h of incubation. The sample on the left side of each gel contains the broad range molecular weight (MW) markers (Bio-RAD, Hercules, CA Min, 2012, unpublished data)...

VI. THE METABOLISM OF RIBULOSE-5-PHOSPHATE 1. Degradation of Ribulose-5-Phosphate... 

The origin of ribose-5-phosphate from ribulose- phosphate has been discussed, as well as the degradation of these pentose phosphates to smaller fragments. The problem of forming ribose phosphate arises in at least two other conditions (1) the utilization of ribose added as a nutrient for microbial growth, and (2) the generation of ribose from nucleosides, either directly by the hydrolytic cleavage of a nucleoside, as in... 

Fig. 43. Oxidative degradation of 1C atom Formation of ribulose-5-phosphate = beginning of the pentose phosphate cycle.

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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