Are there natural or artificial enzymes that break down metals?
Silly answer

There is no enzyme that grinds down an Airstream, but an alligator might do the job!

Serious (but disappointing) answer

In nature, most metals seem to exist as oxides, sulfides, carbonates, halides, etc. and not as bare metals. Consequently, there was/is probably little evolutionary benefit in tackling the bare metals.

Breaking down something to into very small particles, as you described it, is a mechanical process that leaves the oxidation state of the metal unchanged. It is not a chemical transformation.

Nothing cool with enzymes an metals?

There's a lot! Two topics come to my mind:

Chemosynthesis

There's a bunch of bacteria that can oxidize $\ce{Fe(II)}$ to $\ce{Fe(III)}$ 


at different pH levels
in the presence and absence of oxygen
in the presence and absence of light


Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans - look at their family names ;-) - operate at $pH\ &lt; 4$, use the oxygen from the air and munch away pyrite.

Metal hyperaccumulation

Some plants and bacteria harvest enormous amounts of metal ions even from highly contaminates soils and store them. The metals are held by by the action of proteins. If you're interested, here is a freely available article on PLOS One Pathogens with lots of further references.