The conformation which the polypeptide chains assume as a result of hydrogen-bonding is called the secondary structure of the protein. Depending upon the size of the R groups, the following two different secondary structures are possible.
1) $\alpha$-Helix structure
2) $\beta$-Pleated sheet structure

Proteins when subject to external factors such as heat, alkalies, and several electrolytes, they undergo coagulation to form fibrous proteins which are insoluble in water. This leads to change in physical and biological properties of proteins. This type of proteins are called as denatured and process is called as denaturation.

The amino acids which human body can not synthesize and must be supplied in the diet are called essential amino acids. Example, Valine, leucine etc.
The amino acids which are synthesized by the human body are termed as non-essential amino acids. Example, Glycine.