= Golgi cargo processing
{c}
<Golgi cargo processing> includes <enzyme>-mediated remodeling of attached <oligosaccharides>. <Glycosylation> can involve addition of <sugars>, while processing can also remove them. Different <Golgi apparatus> regions have different processing <enzymes>; their <substrate> requirements constrain the order of possible <chemical reactions>.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others/Transport from the ER through the Golgi Apparatus>[Molecular Biology of the Cell, 4th edition by Bruce Alberts and others; Oligosaccharide Chains Are Processed in the Golgi Apparatus; The Golgi Cisternae Are Organized as a Series of Processing Compartments, Figure 13-28]{ref}
An original model makes the importance of ordering explicit. Write a toy <carbohydrate> chain as $XA$, read with its terminal symbol on the right. Define a trimming <function> $T$ that removes a terminal $A$, and an extension <function> $E$ that adds $B$ only when the terminal symbol is $X$. In every other case the corresponding <function> leaves the chain unchanged. Then
$$
XA\xrightarrow{T}X\xrightarrow{E}XB,\qquad XA\xrightarrow{E}XA\xrightarrow{T}X.
$$
In particular, $E(T(XA))=XB$ but $T(E(XA))=X$: the two <functions> do not commute. Each result follows by inspecting the terminal symbol before applying the stipulated rule. These symbols are artificial labels, rather than a natural <oligosaccharide> sequence or measured <enzyme> pathway. The example explains why “two processing steps occurred” does not determine the product unless their order and <substrate> conditions are also specified. In the <Golgi apparatus>, spatial organization can help arrange successive opportunities for processing.
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