How many enzymes are in the human body?
What gene, protein and reaction counts each measure
Matic Broz, PhD6 min
Humans encode about 3,800 enzymes. That is the number of distinct enzyme types written into the genome: the Human Protein Atlas places 3,777 genes in its Enzymes class, and UniProt lists 3,718 reviewed human proteins with a complete Enzyme Commission (EC) number. Between them, those proteins catalyze 1,560 distinct classified reactions.
Search for the question and you will also find 75,000, 1,300 or simply "thousands". These are not competing measurements of the same thing. The smaller figure sits close to the number of reactions, the larger one matches no curated inventory, and neither is a count of enzyme genes. Genes, proteins and reactions each give a different total, for reasons that are easy to trace.
How many enzymes are in the human body?
Humans have about 3,800 enzyme-coding genes. The Human Protein Atlas (version 25.1) places 3,777 human genes in its Enzymes class, the largest of its functional protein classes. That is 19.4% of the 19,442 protein-coding genes in GENCODE v50.[1][2]
The atlas builds that class from three overlapping lists: 3,531 genes from the ENZYME nomenclature database, 509 kinases and 481 peptidases. A narrower atlas class, Metabolic proteins, holds 2,882 genes drawn from the Metabolic Atlas, which follows the network of metabolic reactions rather than catalysis in general.[1]
Protein databases arrive at nearly the same total. In UniProtKB release 2026_03 of 2 September 2026, 4,613 reviewed human entries carry an EC annotation. Of these, 3,718 have at least one complete, four-part EC number, and they come from 3,709 genes. The other 895 carry only partial numbers such as 3.4.24.-, which places a protein among the metalloendopeptidases without naming its exact reaction.[3]
The gene and protein totals differ by less than 2%. The atlas merges gene lists from several sources, while UniProt counts reviewed protein entries with a complete number, and the two handle isoforms, multifunctional proteins and partial annotations differently. UniProt Download returns any of these entries with its EC annotations, so a single enzyme's classification can be checked against the counts here.
How many human enzyme activities are known?
Reviewed human proteins carry 1,560 distinct complete EC numbers, including 9 provisional numbers awaiting formal approval. An EC number classifies a reaction, not a gene or a protein, so this is a count of the different chemical jobs human enzymes do.[3][4]
The ENZYME database gives the same figure from the other direction. Its release of 2 September 2026 holds 6,980 active entries, the classified enzyme reactions known from all organisms, and 1,560 of them link to human proteins. Human enzymes therefore cover about 22% of catalogued enzyme chemistry.[5]
There are more than twice as many enzyme proteins as reactions because many proteins share an EC number. Isoenzymes are distinct proteins that catalyze the same reaction. EC 2.7.11.1, the general serine/threonine protein kinase activity, is assigned to 245 reviewed human proteins, and EC 2.3.2.27, the RING-type ubiquitin ligase activity, to 238.[3][4]
Sharing also runs the other way: 555 human proteins carry more than one complete EC number. Mitochondrial aldehyde dehydrogenase (ALDH2), which clears the acetaldehyde produced from alcohol, carries five.[3] An EC number also says nothing about speed. Turnover rates differ by orders of magnitude between enzymes, as the fastest enzymes show, and DLKcat predicts that rate, kcat, from sequence and substrate.
These totals grow as curators add evidence, not as biology changes. Babbi and colleagues counted 3,428 human Swiss-Prot proteins with complete EC numbers, covering 1,411 distinct numbers, in release 04_2019.[6] The same rule applied to the current release gives 290 more proteins and 149 more activities.
What are the seven types of enzymes?
The seven enzyme classes are oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases and translocases. The first digit of an EC number gives the class, defined by the kind of reaction catalyzed.[4][7]
Transferases lead on both counts. Protein kinases and ubiquitin ligases, which attach phosphate groups and ubiquitin to other proteins to switch them on, off or toward destruction, account for hundreds of human transferases on their own. Hydrolases follow with proteases, phosphatases and GTPases, which use water to cut a bond.[3]
| EC class | Reaction type | Reviewed human proteins | Distinct human activities | ENZYME entries, all organisms |
|---|---|---|---|---|
| EC 1, oxidoreductases | Transfer electrons in oxidation and reduction | 453 | 340 | 2,042 |
| EC 2, transferases | Move a chemical group from one molecule to another | 1,636 | 529 | 2,103 |
| EC 3, hydrolases | Split a bond using water | 1,222 | 446 | 1,377 |
| EC 4, lyases | Break a bond without water or oxidation | 153 | 91 | 783 |
| EC 5, isomerases | Rearrange atoms within one molecule | 159 | 60 | 320 |
| EC 6, ligases | Join two molecules using energy from ATP or a similar donor | 114 | 76 | 257 |
| EC 7, translocases | Move ions or molecules across a membrane | 82 | 18 | 98 |
| All classes | Any classified reaction | 3,718 | 1,560 | 6,980 |
Older textbooks list six classes because translocases became EC 7 only in 2018. Several of the membrane pumps moved into the new class had been filed as ATPases among the hydrolases, although splitting ATP is not their main job.[7]
Why do enzyme counts differ?
Enzyme genes, enzyme proteins, enzyme reactions and enzyme molecules are different units. Current databases support about 3,800 enzyme genes, 3,718 reviewed enzyme proteins with complete EC numbers and 1,560 classified reactions in humans.[1][3]
The figure of 75,000 human enzymes, repeated on question-and-answer pages without a source, fits none of these units. It is more than ten times the 6,980 enzyme reactions classified across all of life and nearly four times the number of human protein-coding genes, so it cannot count distinct human enzyme types.[5][2]
The figure of 1,300, often paired with about 5,000 reactions, is close to the number of distinct EC activities, which was 1,411 in 2019.[6] Read as a count of reaction types, it is out of date. Read as a count of enzymes, it is low by more than half, because isoenzymes push the protein total to more than twice the reaction total.
Larger totals appear when every species counts separately. BRENDA, a curated enzyme database, holds data on 112,288 enzymes from 15,335 organisms, counting the same activity in a mouse, a yeast and a human as three enzymes.[8] Physical enzyme molecules have no fixed total at all: each enzyme type exists in many copies, and its abundance changes between tissues, cell types and physiological states.
The same gap between kinds and copies runs through the whole human proteome. For the question most readers ask, how many kinds of enzyme a human body makes, about 3,800 is the defensible answer, and those enzymes carry out roughly 1,560 classified reactions.
