When both NH2 (amino group) and COOH (carboxyl group) replace hydrogen atoms on the same carbon atom, you have a glycine residue.

This is the simplest amino acid, and it's the building block of proteins. The carbon atom with both the amino and carboxyl groups is called the alpha carbon.

Here's a visual representation:

```

H

|

H2N - C - COOH

|

H

```

Key points:

* Amino acids are the building blocks of proteins.

* Glycine is the simplest amino acid, with only a hydrogen atom as its side chain.

* The alpha carbon is the central carbon atom in an amino acid. It's bonded to an amino group (NH2), a carboxyl group (COOH), a hydrogen atom (H), and a side chain (R group).

Let me know if you have any other questions about amino acids or proteins!