It doesn't affect the nutrition value. The denaturing process just disrupts the tertiary and sometimes the secondary structure of the protein. You are essentially unraveling the protein. The protein loses its function, but you're body digests the amino acids just fine.
Avidin - an enzyme in raw egg-white - interferes with biotin absorption. Cooking, because of what M11 said, denatures this enzyme - meaning it loses its function as an enzyme and thus no longer interferes with the absorption of this nutrient.
That being said, you'd have to knock back a shit-tonne of egg-white for this to be a real problem. Bigger concerns would be the risk of salmonella, and of course, the icky taste.
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