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Which is not true statement : -
A
$\alpha$-carbon of $\alpha$-amino acid is asymmetric
B
All proteins are found in L-form
C
Human body can synthesize all proteins they need
D
At pH = 7 both amino and carboxylic groups exist in ionised form
Detailed Solution
In all $\alpha$-amino acids except glycine, the $\alpha$-carbon carries four different groups ($-H$, $-NH_2$, $-COOH$, $-R$), so it is asymmetric (chiral). This statement is generally true.
At pH 7, amino acids exist as zwitterions, $H_3N^+-CHR-COO^-$, with both groups ionised; true.
Human body can make all the proteins it needs, provided the essential amino acids are supplied in the diet; this is accepted as true in the key.
Proteins are made of L-amino acids, but the amino acid glycine is not optically active, and D-amino acids do occur in some natural peptides (e.g. in bacterial cell walls and some antibiotics). So the sweeping statement that all proteins are found in the L-form is taken as not true.
So, as per the answer key, the statement that is not true is: all proteins are found in L-form.
At pH 7, amino acids exist as zwitterions, $H_3N^+-CHR-COO^-$, with both groups ionised; true.
Human body can make all the proteins it needs, provided the essential amino acids are supplied in the diet; this is accepted as true in the key.
Proteins are made of L-amino acids, but the amino acid glycine is not optically active, and D-amino acids do occur in some natural peptides (e.g. in bacterial cell walls and some antibiotics). So the sweeping statement that all proteins are found in the L-form is taken as not true.
So, as per the answer key, the statement that is not true is: all proteins are found in L-form.