Proteins in human only have alpha amino acids. Why?

Not only that, they are stereochemically of the L-configuration around the alpha carbon. Proteins perform their functions in the cell only by folding into certain shapes. For this, key secondary structural motifs are used, one of which is the alpha helix which I use for illustration. read more

However, only alpha amino acids can be incorporated because the systems of life that chain together amino acids to form polypeptide chains can't work with any other kind. It misaligns the carboxylic acid and amine groups on different amino acids so the ribosome can't join them together. read more

*9 amino acids are essential amino acide:- essential, because these amino acid cannot synthesis in our body, we get it mostly from animal protein. These amino acids are lysine, leucine, isoleucine, histidine, methionine, phenylalanine, methionine, tryptophan, valine. read more

Non-alpha amino acids - the non-proteinogenic amino acids do exist in other biomolecules such as GABA. However these do not end-up in proteins since they exert complexity on the peptide bond formation and the Phi-Psi rotations. read more