In this blog we will discuss a few more aspects of the Enzyme Commission and its classification work that were not covered in the previous blog.
Scope of Enzyme Classification
The classification system used by the EC aims to cover enzymes that fall under one of the following six broad categories:
Class 1: Oxidoreductases
Class 2: Transferases
Class 3: Hydrolases
Class 4: Lyases
Class 5: Isomerases
Class 6: Ligases
As you can see, transporters are not covered by the EC list unless they also catalyze an additional reaction that falls under one of these categories (e.g. the phosphoenolpyruvate-dependent phosphotransferase transporters known as PTS). While peptidases fit under class 3, the Enzyme Commission has limited the classification of peptidases in recent years due to the difficulty in drafting reactions that accurately describe the peptidase specificity.
Principles of Classification
Each top class contains several subclasses. For example, Class 4 contains the subclasses 4.1 carbon-carbon lyases, 4.2 carbon-oxygen lyases, 4.3 carbon-nitrogen lyases, etc. The subclasses, in turn, contain sub-subclasses, e.g. 4.1.1, carboxy-lyases. The sub-subclass in which an enzyme resides defines the first three fields in the enzyme’s EC number. The fourth and last field is simply a serial number within that sub-subclass.
The subclasses and sub-subclasses sometime contain the numbers 98 and 99. In general, when both of those numbers exist under the same parent class, 98 is reserved for well-characterized enzymes that do not fit the other subclasses, while 99 indicates some uncertainty about the enzyme (for example, when the identity of an electron acceptor is not known).
The principles of classification are too complex to describe here. They are described in detail at http://enzyme-database.org/rules.php.
Most of the enzymes fit well in one of the existing sub-subclasses. However, some enzymes catalyze complex reactions that do not fit any particular class. In other cases an enzyme might fit in more than one class. In these cases the commission members need to discuss the issue and decide, and occasionally a new sub-subclass is defined.
What Is The Process of Classifying An Enzyme?
Members of the Enzyme Commission create new entries using an online system Called DraftEnz, which was developed by A. McDonald. The members define the exact sub-subclass to which the enzyme belongs, and the entry receives at this point a temporary internal serial number (e.g. 3.1.3.d). The new entry is reviewed by the other members of the commission, who may suggest modifications to any part of the entry. When a member is satisfied with the entry, he or she may vote for it, and when an entry has received at least two non-author votes, it is ready to move to the next stage, which is internal review.
When a sufficient number of new entries have received the necessary votes, a batch of new entries is moved to internal review, at which time they can be viewed at a dedicated web page, and receive their final serial numbers. All the members of the commission are requested to review them. The internal review process ensures that all members get to review all entries, and problems that were not caught earlier are likely to be spotted.
After one month at internal review, the entries are moved to public review. At this stage the entries are visible to the public at the ExplorEnz website by clicking on the tab “New/Amended Enzymes”. The entries are kept at this stage for another month to allow sufficient time for the community to provide feedback. Once the entries clear this stage, they are moved to ExplorEnz and become official.
In addition to creating new entries, the commission often revises older entries to reflect newer information that has been generated after the entries were created. Existing entries can be revised, deleted, and sometimes transferred to a different EC number. Entries are transferred if new information shows that the reaction catalyzed by the enzyme is different than what was previously thought, requiring the classification under a different sub-subclass, or if new information shows that the enzyme is identical to an enzyme that is classified under a different EC number.
Currently there are 5638 entries in the EC list of enzymes. This number does not include 664 entries that have been transferred and 303 entries that have been deleted.
Since 2010 the commission has created or modified 2221 entries. This is an impressive number for a small group of volunteers, but it is probably a drop in the bucket considering the vast number of well-characterized enzymes that have not been classified yet.
What You Can Do to Help
If you would like to help, it is straight forward to create a new EC entry! You do not even have to suggest the sub-subclass (although you can if you would like). Take a look at a few of the EC entries to get familiar with the format. Then, go to http://enzyme-database.org/forms.php and fill out the form for a new submission. Just make sure you read the information at the beginning of the form, which explains what the requirements are.