Acidity can change protein structure mainly by changing the electrical charges on amino acid side chains. This affects the interactions that keep a protein folded into its normal three-dimensional shape.

When the pH becomes very low, amino acid groups can gain protons (H⁺). This changes their charges and can disrupt important salt bridges and hydrogen-bonding interactions within the protein. As those interactions change, the protein may partially unfold or completely lose its original structure—a process called denaturation.

The effect depends on the protein. Some proteins are relatively stable in acidic conditions, while others lose their structure quickly. At extreme pH, acidity can also promote chemical changes such as hydrolysis of certain peptide or side-chain bonds, particularly under sufficiently harsh conditions.

A simple example is milk curdling. When milk becomes acidic, proteins such as casein lose some of the charge-based interactions that help keep them dispersed. They aggregate together, producing the familiar thick or curdled texture.

So, in simple terms: acidity changes the charges of amino acids → disrupts interactions holding the protein together → alters its shape and potentially its function.

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