Which of the following statements is not correct?

• A
  The $\alpha$-carbon of $\alpha$-amino acids is asymmetric.
• B
  All proteins are found in the $L$-form.
• C
  The human body can synthesize all the proteins it requires.
• D
  At $pH = 7$,both $-NH_2$ and $-COOH$ groups exist in an ionic state.