Hello Therese,
Unfortunately, this is not enough information: the name Jensen is widespread in northern Germany and Scandinavia, as it is derived from the first name Jens and means Jens' son. I have not encountered Jensen in my Jewish research in this area, and the first name Christian is more common among converts.
When I search for Jensen in the Schleswig-Holstein database akvz.de (census database for Schleswig-Holstein and parts of Denmark), there are more than 15,000 hits. Also the name 'Karl Isaak Jensen' does not lead to any hits, as well as research in the Altona database 1845 (Altona was in Schleswig-Holstein, but 'next to' Hamburg).
Kind regards from Northern Germany
Corinna (Wöhrl, née Goslar)