HETATM Records

Of course GRID users also need to study all sorts of molecules as well as proteins, and PDB format provides HETATM records as an easy way for doing this. If the user wanted for some special reason to define his protein structure using H ETATM records, the same atom in the same molecule would appear like this  

Notice the similarities and differences between the ATOM and HETATM records  

All the numbers and symbols are lined up in the same columns as before, but the record now begins with the word HETATM instead of ATOM. This name HETATM is an abbreviation for heteroatom . 

The protein is no longer being treated as a string of amino acids, but as a single molecule which can be given any molecule number (in this case 1). 

The molecule is now called MOL instead of using the specific name LYS which was required by PDB format for lysine in a protein. GRID can accept any three-letter name for a molecule when HETATMs are being used, except names such as LYS which are reserved for known molecules . 

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The HETATM records (Figure 2-115) represent atomic coordinates for atoms within non-.standard grotips (water molecules and atoms presented in HET... 

The sequence of ATOM records in a molecule is specified by PDB format. However HETATM records can be listed in any sequence, and the nitrogen has been moved to the 25th row of the new HETATM file. There is nothing special about its new position, and the user could just as easily have moved it to the first or last row of the file, or left it where it was. 

Three record types are used to group other records TER indicates the end of a chain, while MODEL/ENDMDL surround groups of ATOM, HETATM. TER and similar records. 

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