What Software could I use to create Diagrams of Biochemical Processes like these?
By posting to this forum, I presume the question is whether the illustrators of this and other biochemistry books used software specifically designed for the purpose of drawing reaction pathways, or, if not, whether such software exists. From my experience of using such illustrations in my own teaching over the years, I believe that the answer is probably no (although there are a couple of what I would consider ‘almosts’ mentioned below.) The main reason I would not expect this is that it would not seem commercially viable (biochemistry teachers and book illustrators are hardly a huge market) and you can get by with more generic pre-existing applications.

The class of application I consider best for this is termed a vector graphics application (popularly known as a ‘drawing’ application), as opposed to a bitmap or raster graphics application (popularly known as a ‘paint’ application). Although today there are applications that combine both functions, vector graphics applications have the advantage that they are more easily editable and they scale to any print resolution. The professional applications in this area can be very expensive and have a steeper learning curve than bitmap graphics applications (such as Adobe Photoshop). Moreover, they may lack features needed for biochemical use. It is therefore be worth exploring cheaper offerings, or investigating specialist vector graphics applications more focused on engineering or computing diagrams.

For biochemical work I personally would require software to be able to provide the following:


Controllable geometric lines (Bezier curves)
Customizable arrowheads
Text control that at least allows subscripts and
superscripts, and ideally both simultaneously, e.g. for a charged
phosphate species.
Rotatable text (as in the upper example)
Basic graphic primitives (as in the lower example)
Export to vector formats accepted by other applications (PostScript or PDF) to allow output in formats and at resolutions required for print publication.


How do different applications satisfy these criteria? They all support basic graphic primitives, and most allow export at least as PDF, if not the more flexible PostScript.

Surprisingly the creation of a line with an arrowhead is not straightforward as one is dealing with a combination of two separate objects. Although all applications tend to provide straight lines with arrowheads, curved lines with arrowheads tend to be absent from the general applications (even the most sophisticated) because a curved line is not really a line as such, but is generated as an oval segment with a stroke (visualization of its drawn edge) but no fill (the body is transparent). I have tended to use this sort of application (going back to MacDraw II of the Classic Mac) and fudge the arrows on them by combining them with straight lines of the same length as the arrowhead. However they are provided by more specialized diagramming software like OmniGraffle (mentioned in another answer) or the dedicated ChemDraw (which I find too limiting to use other than for export). 

Text handling is the other problem. Placing a superscript directly above a subscript is a page layout operation that only dedicated mathematical software (e.g. MathType) is likely to support. Microsoft Word’s equation editor will do this, but export is only as PDF, and import into other applications varies in effectiveness. If you work in a generic application you will have to fudge it yourself with separate text boxes.

What are the choices for generic vector drawing applications? Adobe Illustrator is the most sophisticated, is used by most professional illustrators, but is very expensive (it is now subscription only). Other vector drawing applications that I am aware of but have not used include the commercial Corel Draw (cheaper than Illustrator) and the even cheaper Affinity Designer. As mentioned in a comment, there is also the free application, Inkscape. Free trials are available with most applications, so I would recommend seeing what is the best solution for your personal situation.

Addendum

I have just had a quick look at BioRender, which was mentioned in another answer. It has many different modules, including ones for biochemistry (including a Glycolysis pathway template) and seems quite attractive. However if you want to use the results for publications you need a $35 per month personal academic subscription (or your institution to buy a site license). I guess you tend to get what you pay for.