system
Given the following post, respond as if you're commenting on Reddit:
human
Is ethanol the only alcohol that humans can tolerate because of enzyme specificity or the relative harmfulness of the products?

Humans can only tolerate ethanol. Ethanol and NAD+ are converted to acetaldehyde and NADH by alcohol dehydrogenase. Acetaldehyde and NAD+ are then converted to ethanoic acid and NADH by aldehyde dehydrogenase. 

So is it that one (or both) of these enzymes is specific to two-carbon molecules, or is it that the by-products from a reaction with  methanol, isopropyl alcohol, etc. are more potent in their toxicity?

EDIT: I am guessing the former, because enzymes essentially lower the energy of the transition state by making hydrogen bonds and forcing molecules into a specific conformation. This means there isn't room for extra carbons because hydrogen bonds can only occur within a window of ~1 Angstrom (it's like 2-3 Angstroms away for an H-bond, I think). Wikipedia is not specific enough and the articles I'm reading don't address the physiology. Correct me on any of this.
gpt
Alcohol dehydrogenase is a rather non-specific enzyme.  It's primary physiological role isn't to metabolize ethanol, at least not in humans.  Most any straight chain alcohol can fit into the active site and get oxidized to the corresponding aldehyde.  Even something as large as retinol can act as a substrate to this enzyme.  

Aldehyde dehydrogenase is a broad family of enzymes, some of whose members are specific, and some that are non-specific.  The enzyme ALDH2 does work quite well in the oxidation of acetaldehyde, but there is evidence that it utilizes something as large as 3,4-dihydroxyphenylacetaldehyde (a metabolite of dopamine) as a substrate, too.

As for the toxicity aspect, acute alcohol toxicity can occur for nearly any alcohol due to their depressant effects.  The aldehyde metabolites are also toxic, and are responsible for alcohol flush and hangover symptoms.  The formaldehyde produced from formaldehyde metabolism is very toxic (LD50 200 mg/kg), but the acetone produced from isopropanol metabolism isn't significantly more toxic than the alcohol (both about 5000 mg/kg).  For comparison, the LD50 of ethanol is 7100 mg/kg, while that of acetaldehyde is about 3500 mg/kg.