All proteins are

When catabolism of proteins exceeds synthesis of proteins,then:

Which of the following exists as a Zwitter ion?

Find the correct option for the sequence given below:
$(1)$ Alanine $\rightarrow$ Valine
$(2)$ Asparagine $\rightarrow$ . . . . . .
$(3)$ Aspartic acid $\rightarrow$ Glutamic acid
$(4)$ Arginine $\rightarrow$ Histidine

Which of the following groups in the structure of an amino acid is acidic?

Amino acids with polar and negatively charged '$R$' groups are ..........