Since amino groups are present as their ammonium ion conjugate acids in neutral solution and carboxylic acids are present as carboxylate ions, the positive charge of an ammonium ion on one chain is attracted to the negative charge of a carboxylate ion on another. A salt brigde is an ionic bond between ions that are part of alarger covalently bonded unit. If it is an enzyme, its catalytic activity is destroyed. Often, the protein becomes insoluble and precipitates, as happens when an egg is cooked or the casein from milk separates as acid is introduced. There are several classifications of proteins.
Simple proteins are all protein, with no non-protein comoponents.
Conjugated proteins are covalently bonded to some non-protein materials. Examples include glycoproteins which have some carbohydrate portions, lipoproteins, which include lipids such as cholesterol, metalloproteins which include metal ions, etc. This classification has to do with shape, and the terms are pretty much self explanatory. Fibrous proteins are found in fibers and are insoluble. Globular proteins are soluble and are thus less obvious.