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I've looked in several sources, but I'm still confused. This is what I have so far:

  • A phosphotransferase catalyzes the addition of a phosphate group.
  • A kinase is a type of phosphotransferase that transfers a phosphate group from ATP to a substrate.

  • A phosphorylase is a type of phosphotransferase that catalyzes the addition of a phosphate group from an inorganic phosphate (HPO4) to a substrate.

  • A hydrolase catalyzes the hydrolysis of a chemical bond.

  • A phosphatase is a type of hydrolase that removes a phosphate group.

Is this right?

EDIT: Also adding to the confusion is the enzyme Phosphorylase kinase, which is the kinase that phosphorylates the enzyme Glycogen phosphorylase, which hydrolyzes glycogen.

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  • $\begingroup$ Phosphatase uses water to remove a phosphate group, while Phosphorylase uses phosphate group to remove the phosphate group $\endgroup$
    – vee kay ramadesikan
    Commented Sep 2, 2017 at 0:34

2 Answers 2

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Don't get caught in the linguistic trap of attaching too much significance to the precise meaning of any one piece of scientific jargon. Nomenclature, the system of naming things, utterly sucks in biology, and it sucks even harder for protein biology. There is no one universally accepted classification and naming system for proteins, although there have been plenty of attempts.

For example, phosphotransferases are classified as EC number 2.7. However, Glycogen Phosphorylase is classed under 2.4, which might make you think "well, clearly kinases are enzymes that transfer phosphate groups from organic substrates such as ATP and are classed as 2.7, whereas phosphorylases are enzymes that transfer from inorganic substrates and are therefore classed as 2.4". But wait! RNase PH is a phosphorylase that's classed under 2.7 and uses inorganic phosphate, so um... yeah.

In any case, don't get too hung up on the nomenclature. Unless you're working in an enzymology lab just say "kinase" and everyone will know what you're talking about.

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  • $\begingroup$ I'm certainly not an expert on this but it seems to me that glycogen phoaphorylase could also be classified as a phosphotransferase (2.7). However, since there's already a glycosyltransferase group (2.4) it was classified there. There is no such group for ribonucleotidyltransferases which is why RNase PH was put in 2.7. This is perhaps a drawback of the EC system in that one catalysed reaction could fit two different groups. Even though glycogen phosphorylase is classified as a glycosyltransferase by EC, it is still also a phosphorylase. $\endgroup$
    – canadianer
    Commented Oct 31, 2014 at 2:56
  • $\begingroup$ @canadianer That's true. My point was more about the futility of trying to define a short nomenclature for the set of all enzymes. Their complexity defies any simple classification. Save the jargon for when you have specifics to talk about, and talk about generalities in general terms. $\endgroup$
    – tel
    Commented Oct 31, 2014 at 3:34
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As @tel said, there is no clear difference available since there is no international authority which manages such things (like IUPAC) when it comes to biological terms. However, these terms can be understood and differentiated a bit more easily, as long as you don't get into deeper details.

Posphotransferase is a kind of transferase that transfers a phosphate group from one compound to another. The type of substrate remains unspecified (dictionary.com).

Kinase is a type of phosphotransferase that transfers a phosphate group from high-energy phosphate donating molecules (usually ATP) to anothrr molecule. Here, one substrate is specified (Wikipedia).

Phosphorylase is an enzyme that adds an inorganic phosphate ($\ce{PO_4^{3-}}$) to a substrate molecule. The inorganic substrate is not transferred, it is directly added to a molecule (Wikipedia).

Hydrolase is an enzyme that hydrolyzes a substrate i.e. breaks a covalent bond in a molecule and adds water to the resulting molecules. Its like $$\ce{A-B + H_2O \rightarrow A-OH + B-H}$$ where A-B is a single substrate molecule (Wikipedia).

Phosphatase is a type of hydrolase that hydrolyzes the bond between a molecule and phosphate group, resulting in the formation of inorganic phosphate and an alcohol. It is like $$\ce{A-PO_4^{2-} + H_2O \rightarrow A-OH + HPO_4^{2-}}$$ where A becomes an alcohol (Wikipedia).

PS: I am not saying that these are the universally accepted definitions of the corresponding enzymes. These are just examples to help you better understand the meaning of these terms. However, you can always find exceptions to these definitions.

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