I've been taught 2 things that seem to contradict each other:
If this is the case, then cooked meat must have very little usable protein, which is clearly not the case.
How does protein in a fully cooked food, e.g. chicken/pork/eggs survive the cooking process in a usable form?
Note I'm not asking about burnt meat, browning or flavour reactions, or rare meat e.g. a medium/rare cooked steak.
Structural proteins in foods, i.e. albumin in eggs, myosin in muscle meats, gluten in wheat, are formed by amino acids in complex structures. Proteins are folded and clumped chains of peptides, and peptides are chains of amino acids. Your body uses stomach acid and proteases (enzymes) to break down proteins in digestion, but this requires time and energy (stomach churning, body heat).
The amino acids are what your body requires. They're used to build more complex proteins. Cooking gives a head start in un-clumping/unfolding proteins, and in some conditions into peptides and amino acids, that are easier for your body to digest making them more 'bioavailable'.
Some of these amino acids can be produced by your body using nitrogen compounds from breaking down other amino acids and nutrients. The ones your body can't produce on its own have to be obtained in the proteins you eat. These are called 'essential amino acids'.
Different protein sources have different amino acid compositions, giving them different 'protein values' in how complete the range of essential amino acids are available; i.e. collagen/gelatin is a common structural protein that provides great texture for sensory applications, but the protein value is 0 for regulatory labelling requirements in some countries since it is missing an essential amino acid.
The denaturing of proteins refers to their physical structure, not their nutritional value. This might help. As is detailed in the link, heat, acid, salt, alcohol, and mechanical agitation can cause proteins to denature. This denaturing is when the folded strands of protein unfurl into a long strand of amino acids.
Another way to think about proteins and tissue is like lego blocks. Imagine the tissue or steak you eat is a well-crafted lego structure, for example a building. Heating it might break apart some of the corridors, or remove a floor from the other floor but the individual lego pieces are still mostly intact. What your body needs from the protein you eat are individual amino acids, legos in this case. Your body must still further break down the corridors/building floors (steak) into individual legos (amino acids) which your body can absorb in your small/large intestine.
In this case, cooking might slightly break down your building (steak) but it won't break it down all the way to the level of individual legos (amino acids) which your body needs.
Another analogy would be, if you smash up a car engine so it cannot run anymore, the engine is 'destroyed' yet you still have all the scrap metal. And as our body wants protein in the smallest possible scraps (amino acids) cooking, denaturing, smashing up, is useful. Becuase proteins have complex strucures that enable them carry out their purpose, changes to the structure 'destroy' the protein, i.e. destroy their ability to carry out their purpose. Note that denatured alcohol is different. the structure of alcohol is too simple to destroy (unfold). Denatured alcohol cannot be used for it's main purpose (getting you nice and drunk) because something has been added that makes it undrinkable (like the hideous taste of some alcohol gels if you use them, then eat food then try to lick your fingers).
