Can someone kindly explain upstream vs. downstream?

Question

[vxluwidrzzqvjwnwix]

Hi everyone,

Can someone please explain what "upstream" and "downstream" mean when it comes to pathways? For example, if someone's looking at downstream pathways that a protein is linked to, what does this mean, and why is that important?

Answer 1

[lieven.sterck]

if a protein catalyses a certain reaction (A) and the resulting compound serves as input for another reaction in a different pathway (B) you denote it as downstream of the pathway where this first protein (or reaction) is part of. Pathway B needs pathway A to function, it is thus downstream in the chain of reactions

other way around, if the input for a certain reaction is derived from a reaction/modification from another pathway it's upstream.