eg : 2-Amino-5-guanidinopentanoic acid (arginine) , in this case prefix amino is put rather than the suffix amine . Why does this happen ?
Am I missing something ?

Take glycine as an example (because I can remember it's structure without looking it up)

It's generally called aminoethanoic acid.

If you wanted to call it (something)ethylamine what would you put in the brackets there?

But, essentially, it's just convention.

Take glycine as an example (because I can remember it's structure without looking it up)

It's generally called aminoethanoic acid.

If you wanted to call it (something)ethylamine what would you put in the brackets there?

But, essentially, it's just convention.

Is putting amino in the prefix rather than in the suffix a convention only for amino acids or even compounds ?

The longest chain is 5 carbon atoms long, pentatonic acid.

The substituents are then included in alphabetical order.

The numbers, the sum of which must be as low as possible.

So , the anime group is a substituent rather than a functional group ?

It is both.

Conventions in nomenclature mean that certain functional groups take priority over others. We use that system to keep things consistent. If you have a molecule with two or more functional groups, such as a carboxylic acid and an amine, the group with the highest priority (the carboxylic acid in this case) becomes the parent functional group and is the one that ends up as the suffix.

 

This is a good list of priorities, for future reference: http://www.laney.edu/wp/abraham-reyes/files/2011/09/Priority-of-functional-groups.pdf