Plant protein v animal protein?

Iguanas are herbivores. Here lately it seems like there is a lot being said about how harmful it is to feed an iguana any animal protein, because of possible resulting kidney problems.
What exactly does this mean? I've tried to research this claim, but meet with the same thing at every site: a warning, tapering off to a vague mention of kidney problems. What is it EXACTLY at the cellular level, that overwhelms the kidneys of an iguana consuming animal protein?

The common claims that you shouldn't feed your iguana ANY animal protein for fear of kidney damage is an overreaction to a problem.
It's not the animal protein that is the problem per se, it's the concentration of it as compared to plants.
The ratio of amino acids are different in the two sources, but essentially they are similar. The problem lies with meat having far more of it.

To understand what effect this has on the kidneys you need to understand how the body of the iguana processes the protein (I hope I'm not over explaining this and telling you stuff you already know).
An iguanas digestive system is optimized to process plant matter and the corresponding lower levels of plant based proteins. When trying to process meat, the digestive system can't fully metabolize all that excess protein that meat contains. As the extra protein is broken down, the iguanas body has to convert a portion of it to uric acid.

Since the high levels of protein can't all be used, the amount of waste material generated from meat far exceeds that produced by plant matter.
The kidneys come into play to remove that waste from the system. It's this over working of the kidneys that contributes to their damage, and in the long term their failure. The animal protein doesn't actually damage the kidneys directly on a cellular level, it just forces them to work harder to the point of damaging them.
As an aside, when the levels are such that the kidneys cannot remove all the uric acid, it can lead to deposits of it in the body causing gout as well.

Some people talk about animal based protein in such a way as to equate it to toxic poison in an iggys diet. This isn't actually the case. They can consume animal protein in small amounts. The problem is, you don't know what constitutes a small amount or at what point you're risking damage.
In the end though, an iguana has no inherent need to consume animal protein at all, so there's no reason to risk it. A properly balanced totally plant based diet is all they require.
That being said, if your iguana happens to like cheese or what have you, you're not going to risk harming him if you give him a little of it once in a while.

Great explanation, and just what I was looking for thanks.
Andso somehow lizards like iguanas and others like, let's say, bearded dragons, must have developed along completely different lines, perhaps in response to their environments?
I'm going to be careful and exclude animal protein for now at least until I refresh my memory as to what plants also have higher quantities of protein.