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POSTTRANSLATIONAL MODIFICATION Posttranslational modification can affect membrane association by prenylation (adding C15 or C20 unsaturated hydrocarbons) or fatty acylation (C14 or C16). Glycoproteins and glycolipids on the exterior face of the membrane have carbohydrates attached. Some proteins can be posttranslationally modified by the addition of prenyl groups. Prenyl groups are long-chain, unsaturated hydrocarbons that are intermediates in isoprenoid synthesis. The farnesyl group has 15 carbons, and the geranylgeranyl has 20 carbons.

If you make a mistake here, all it will do is change the sign of ⌬G, not the magnitude of the number. So be sure that you determine if each ⌬G makes sense. 95 kcal/mol. But this must be wrong—we know that with a higher concentration inside, the Naϩ would like to move down its concentration gradient. Since we’re calculating the ⌬G for moving from outside to inside (we assumed that), then the ⌬Gconc must be positive. 95 kca/mol. 95), the Naϩ will move in the direction we assumed (outside to inside).

This is near the potential that the cell normally maintains and denotes that there are more negative ions inside the cell than outside. Let’s also assume that the concentration of Naϩ is 20 mM outside and 100 mM inside. Now, if we were to punch a hole in the membrane that would allow Naϩ to move (a channel), which way would it move? The signs of ⌬G are dictated by what you call inside and outside, so let’s try to figure out the magnitude of ⌬G and determine the sign of ⌬G by intuition. Because the cell is negative inside, the membrane potential would tend to cause the Naϩ to move from outside to inside (ϩ likes Ϫ).