organic chemistry - Why is methanol toxic?

There are two points of view for the answer of this question:

• The biological view, the only one that I faced during my research, states that since it can trigger perilous conditions like metabolic acidosis, cause a neurologic sequelae, etc. methanol is toxic.
  


• The chemical point of view in an answer, which is the one I seek, is the reactions that occur in the body with methanol as a reactant.
  

Why should it be a reaction? Because I assume something will be toxic for humans when it reacts and disables some of their bodies' vital compounds; e.g.: Hydrogen cyanide reacts with enzymes, renders their active sites useless, and finally causes heavy reduction in bio-chemical reactions that results in quick death.

So, the question is: What reactions use methanol in the body that makes it toxic for us?

Answer

Methanol isn't particularly toxic in and of itself, although it's no walk in the park.

If methanol flowed through the body without being broken down, it would cause roughly the same kind of harm as ethanol, i.e. intoxication.

The real culprit is one of its metabolic products, methanoic acid, also known as formic acid.

To understand how formic acid, present as the formate ion, is toxic, we look to Wikipedia:

Formate is toxic because it inhibits mitochondrial cytochrome c oxidase, causing the symptoms of hypoxia at the cellular level, and also causing metabolic acidosis, among a variety of other metabolic disturbances.

Edit: As DavePhD points out, an intermediate product in this process is formaldehyde, or methanal. While formaldehyde is also toxic, it is rapidly metabolized to methanoic acid.

Reedit: The deeper, more historical reason that this happens is that methanol isn't readily available in nature, meaning that few species have developed biochemical tools to deal with it. There simply hasn't existed an evolutionary pressure to deal with methanol.