Which is not a true statement ?

Options

A. `alpha` - carbon atom of `alpha` - amino acids is asymmetric except in glycine
B. All proteins contain `alpha` - amino acids of L - configuration
C. Human body can synthesize all proteins they need
D. At pH = 7 both amino group and carboxylic group exists in the ionised form

Explanation

To determine which statement is not true regarding proteins and amino acids, let's analyze each statement step by step.

### Step 1: Analyze the First Statement
**Statement:** The alpha carbon atom of alpha amino acid is asymmetric in glycinin.

- **Explanation:** Alpha amino acids have a central carbon atom (the alpha carbon) that is bonded to four different groups: an amino group (NH2), a carboxyl group (COOH), a hydrogen atom (H), and a variable side chain (R group). This makes the alpha carbon asymmetric (chiral) in most amino acids, including glycinin.

- **Conclusion:** This statement is **true**.

### Step 2: Analyze the Second Statement
**Statement:** All proteins contain alpha amino acids of L configuration.

- **Explanation:** While most naturally occurring amino acids in proteins are of the L configuration, there are some exceptions. For example, certain bacteria and some proteins may contain D-amino acids. Therefore, it is incorrect to say that all proteins contain only L configuration amino acids.

- **Conclusion:** This statement is **not true**.

### Step 3: Analyze the Third Statement
**Statement:** The human body can synthesize all proteins they need.

- **Explanation:** The human body can synthesize many proteins, but it cannot synthesize all amino acids. There are essential amino acids that must be obtained through diet. However, the statement refers to the synthesis of proteins, which is true in the context that the body can produce proteins from the amino acids it has.

- **Conclusion:** This statement is **true**.

### Step 4: Analyze the Fourth Statement
**Statement:** At pH equal to seven, both amino group and carboxyl group will exist in the ionized form.

- **Explanation:** At physiological pH (around 7), the carboxyl group (COOH) tends to lose a proton (H+) and exists as COO^-, while the amino group (NH2) tends to gain a proton and exists as NH3^+. Thus, both groups are indeed ionized at this pH.

- **Conclusion:** This statement is **true**.

### Final Conclusion
After analyzing all statements, we find that the second statement is the only one that is not true.

**Answer:** The statement that is not true is: "All proteins contain alpha amino acids of L configuration."

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