Waiting for the protein and chemistry divas to jump in. But my two cents:
Any protein can be hardcore or moisturizing, depending on the formulation, concentration and percentage of protein
Keratin which is suppose be pretty strong protein can be in a very moisturizing & hydrating product if the keratin is sufficiently "hydrolyzed" (which means broken down into the smallest pieces with water) or are "amino acids" (don't know what that means, but amino acids usually means the product can penetrate) and combined with moisturizing & hydrating ingredients.
My beloved nexxus humectress is most definitely a super moisturizing condish, even tho it has keratin amino acids as an ingredient. I have left it on 12 hours or more overnight and it never leaves hair hard, indeed, overnight left my hair too soft and perhaps even overmoisturized. Soooo my personal experience is, proteins can be moisturizing or can assist with binding in moisture as others have already stated. Note the many many luscious moisturizing ingredients in humectress: Water (Aqua), Stearalkonium Chloride, Glycerin, Cetyl Alcohol, Mineral Oil, Cetearyl Alcohol, Wheat Flour (Triticum Vulgare) Lipids, Tetrahexyldecyl Ascorbate, Alpha-glucan Oligosaccharide,
Keratin Amino Acids, Panthenol, Tocopheryl Acetate, Magnesium Citrate, Manganese Citrate, Avocado (Persea Gratissima) Oil, Soybean (Glycine Soja) Sterol, Ceramide 3, Butyl Methoxy-dibenzoylmethane, Ethylhexyl Methoxycinnamate, Myristic Acid, Palmitic Acid, Stearic Acid, Sodium Chloride, Polyglyceryl-3 Distearate, Polysorbate 60, Polysorbate 20, Potassium Sorbate, Guar Hydroxypropyltrimonium Chloride, Petrolatum, Stearyl Alcohol, Disodium Edta, Dmdm Hydantoin, Iodopropynyl Butylcarbamate, Peg-40 Hydrogenated Castor Oil, Citric Acid, Phenoxyethanol, Methylparaben, Propylparaben, Fragrance (Parfum), Red 4 (Ci 14700), Yellow 5 (Ci 19140), Blue 1 (Ci 42090), Red 33 (Ci 17200).
Wheat protein which is supposed to be "moisturizing" can be hardcore if used in a concentrated formula with no moisturizing ingredients. I have a bottle of wheat protein from
www.ingredientstodiefor.com and used it straight up and it left my har hard -- but once I rinse, hair felt good. I didn't try it again because while my loves proteins in general, I don't like the hardcore stuff that leaves my hair hard like that. I add the wheat protein to products I want to tip into being more proteiny, just like I add glycerin or honey to tip into being more moisturizing
Okay, I have exhausted my limited knowleged on this matter . . .