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The follow shows a protein pathway pathway.

enter image description here

The question is "where does galactose (GAL) first occur" i.e. which block above is galactose first contained in?

A transferase is defined as (wikipedia):

any one of a class of enzymes that enact the transfer of specific functional groups (e.g. a methyl or glycosyl group) from one molecule (called the donor) to another (called the acceptor)

This definition of transferase is confusing as:

  • The question doesn't show any functional groups does it?
  • From it, I can't figure out whether the galactose transferase is the molecule that is the donor or acceptor

I can't decide if galactose first occurs in S (as a donor) and is transfered into an acceptor upon reaching T, or if another donor forms galactose (as an acceptor) after S, causing T to contain galactose.

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    $\begingroup$ Can you please explain this diagram and cite its source? I think it depicts the processing of a glycoprotein and shows how different sugars are added stepwise. $\endgroup$
    – WYSIWYG
    Commented Sep 20, 2016 at 11:07
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    $\begingroup$ Yes, this is a biochemical pathway showing a scheme whereby proteins in the endoplasmic reticulum and Golgi apparatus become glycosylated on their way to the cell surface. The square boxes labelled with letters represent a generic protein substrate as it moves along the pathway. The arrows between the boxes are labeled with the names of the enzymes that are performing the post-translational modifications. Galactose is a carbohydrate, specifically, a hexose with a 6-carbon backbone. It would be one of the substrates for the Gal transferase enzymes. $\endgroup$
    – RosieF
    Commented Sep 20, 2016 at 12:06
  • $\begingroup$ You need to google for galactosyl transferases and find out what kind of reaction they catalyse. Then, if S and T represent proteins with growing polysaccharide chains (as @RosieF states, although I personally would have no idea) you should be able to work this out for yourself. But I think the question is unclear because of the schema and am voting for it to be closed. $\endgroup$
    – David
    Commented Sep 20, 2016 at 22:31

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