You won't be able to directly use your PuTTY's key in Linux's OpenSSH because the keys are of different format. You'll need to first convert PuTTY's key to OpenSSH's key format by following these steps;

1. Download PuTTY Key Generator (puttygen.exe) from the following URL

   https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html

2. Launch the program and click on the Load button and select your PuTTY's key file, which normally ends with .ppk extension.
   
   
3. Enter the key's passphrase if prompted.
   
4. Details of the imported key will be displayed.
   
5. Go to Conversions → Export OpenSSH key from the menu.
   
6. Save your converted key to a new file and location.