Glycoproteins - Hughes R. 1985

Biosynthesis
Control of glycan biosynthesis
Metabolic control

A resting Cell, such as a quiescent lymphocyte or a serum-starved fibroblast, maintains a baseline rate of METABOLISM/35.html">Protein Biosynthesis and glycosylation. Both of these processes can be stimulated, for example, by adding a mitogen or serum. How is it that a cell like a lymphocyte can dramatically ramp up glycosylation post-stimulation to the high levels seen during immunoglobulin biosynthesis? Does this resemble the way hormone-responsive Cells (such as thyroid cells) mobilize their metabolism to produce a specific terminal glycoprotein (such as thyroglobulin)? There are several plausible mechanisms. It is conceivable that the intracellular concentrations of glycosylating Enzymes and active sugar intermediates under resting conditions are already at saturation levels. In this case, the glycosylation of an increasing number of newly synthesized Polypeptides would be coordinated accordingly. However, existing evidence suggests that glycosylation reactions are regulated by the intracellular availability of dolichol and dolichol phosphate. The synthesis of dolichol phosphate is part of the common biosynthetic pathway for Cholesterol and ubiquinone (Fig. 3.15), and the enzyme hydroxy-3-methylglutaryl-CoA reductase, which catalyzes The formation of mevalonic acid (Fig. 3.15, step 2), serves as a key regulatory checkpoint in The biosynthesis of both compounds. An inhibitor of this enzyme, 25-hydroxycholesterol, suppresses the synthesis of cholesterol and dolichol phosphate while reducing protein glycosylation, at least in certain cell types [50]. A similar inhibitory effect is caused by mevastatin (compactin), a potent inhibitor of hydroxy-3-methylglutaryl-CoA reductase derived from Penicillium citrinum [51]. Interestingly, adding compactin to developing sea urchin embryos blocks normal Gastrulation, whereas supplementing with exogenous dolichol rescues development, presumably by directly supporting glycosylation. In the Liver, excess cholesterol suppresses the synthesis of hydroxy-3-methylglutaryl-CoA reductase; however, this enzyme cannot be the primary rate-limiting step in hepatic cholesterol synthesis because The conversion of farnesyl pyrophosphate to cholesterol is also suppressed (Fig. 3.15, step 4). As a result, there is an increased flux of farnesyl pyrophosphate toward dolichol phosphate (step 5) and ubiquinone (step 8), which stimulates protein glycosylation [52]. Under normal conditions in the liver, only about 1/100 of these precursors is channeled into dolichol phosphates compared to cholesterol. Therefore, even when hydroxy-3-methylglutaryl-CoA reductase is partially inhibited, sufficient farnesyl phosphate is still produced to sustain dolichol phosphate synthesis.

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Fig. 3.15. Biosynthesis of Cholesterol and dolichol. CoA — coenzyme A.

Taken together, these findings point to an important Metabolic Control of protein glycosylation operating at the level of dolichol phosphate formation. Because dolichol phosphate availability limits The rate of glycosylation, there may be additional regulation governing the conversion of dolichol pyrophosphate (Fig. 3.15, step 9) back to dolichol phosphate via specific Phosphatases and a CTP-dependent kinase, which act independently on distinct pools of free dolichols or dolichyl esters.