Depending on whom you speak with, people have different views depending on the structure of the organization.
The first of these views is that when the business has an idea, it engages with the PMO to draft a business case, engage with the appropriate stakeholders, etc. This is generally seen from a cost perspective.
The second of these is that when the business has an idea, it engages with the Enterprise Architecture Office to assess the current state, establish the future state, identify the gaps and then engage with the PMO. This is generally seen from a capability perspective and then cost i.e.
I am however unclear on the benefits and disadvantages of each and when each model is appropriate.