It doesn't matter that much if you don't have a biochemical background when applying for biochemistry. I know a lot of guys studying biochemistry who do not have such background before. They studied either agronomy or animal sciences before.
What matters is whether what you're doing now is matching the professors' field. Biochemistry professors do not necessarily solely concentrate on pure molecular studies like proteins structure, DNA and genes, they may apply this to certain kind of human, plant, animal or microbe research. So you will have a lot of chances to match them even if you're not a protein or gene guy.