HFCS is a mixture of fructose and ~~sucrose~~ glucose. If it was pure fructose it wouldn't be as an effective of cane sugar replacement.

Fructose mixed with glucose, not sucrose (the same typo is in several comments here). Sucrose is composed of glucose and fructose linked together.

When corn starch is processed, the produced syrup is nearly 100% glucose. Because fructose tastes sweeter, lots of corn syrup is further processed, converting much of the glucose into fructose, hence the name high fructose corn syrup.