It’s simple to drag and drop bibliographic references from Zotero to Authorea. Just make sure you set your Zotero default Export Format to BibTeX.
In Authorea’s editor, go up and click the Cite ’plus’ button. Under DOI/BibTeX, there is a box to drag your citation from your list in Zotero (if you don’t yet have BibTeX as your default export option, see below for a quick guide) (and here’s a cool reference on designing new enzymes! (Giger 2013)).