shell - Sed replace line stored in a variable

I need to find a line that starts with mysql.unmetric.twt.url and replace the entire line with line stored in the variable ip.

ip="mysql.unmetric.twt.url=jdbc:mysql://ec2-a-b-c-d.compute-1.amazonaws.com:3306/unmetric? jdbcCompliantTruncation=false&useUnicode=yes&characterEncoding=UTF-8&useOldAliasMetadataBehavior=true"

How can this be achieved using sed?