* Theoretically: The number of possible protein types is astronomically large. With 20 amino acids, even a short protein of 100 amino acids can have over 10^130 (that's a 1 followed by 130 zeros!) possible combinations.
* Practically: The number of known protein sequences in databases is constantly growing, but is estimated to be in the millions.
* Functionally: Proteins are grouped by function: enzymes, antibodies, structural proteins, hormones, etc. There are many hundreds of distinct functional categories.
* Structurally: Proteins can also be classified by their 3-D shape (alpha helix, beta sheet, etc.). There are many different structural motifs.
So, to answer your question:
* There are millions of known protein sequences.
* There are many hundreds of distinct protein functional categories.
* There are many different structural protein motifs.
The number of unique proteins in a single human cell is estimated to be in the thousands, and the total number of different protein types across all organisms is likely to be in the billions.
It's important to remember that this is a constantly evolving field! New protein sequences are being discovered all the time, and our understanding of protein function and structure is constantly improving.
