It is often observed that Gly residues are conserved in proteins to a greater degree than other amino acids. From what you have learned in this chapter, suggest a reason for this observation.

When we take a closer look at proteins, we notice that certain amino acids are kept unchanged through the evolutionary path of a species more frequently than others. This preservation of specific amino acids, like Glycine (Gly), is referred to as 'amino acid conservation.' But what makes Glycine so special that it is so often conserved?

Glycine's minimal side group - a single hydrogen atom - is the key reason for its conservation. Its small size allows it to occupy positions in the protein structure that are geometrically impossible for bulkier amino acids. This is particularly crucial in parts of proteins that require tight turns or coils, known as 'turns' in the protein backbone. Here, Glycine acts like a hinge, enabling the protein chain to bend and twist as needed, which is essential for the overall three-dimensional shape and function of the protein. Therefore, substituting Glycine with another amino acid could potentially destabilize the protein's structure or impair its function, making Gly's conservation vital to maintaining protein integrity over generations.

Proteins are remarkable biological polymers, adopting intricate shapes that are pivotal for their diverse functions. The flexibility of a protein's structure is directly linked to its ability to perform its specific tasks within the cell. Flexibility here refers to the protein's capability to undergo conformational changes in response to various biological stimuli.

Within this context, Glycine is a standout contributor to protein flexibility. Due to its unencumbered structure—a hydrogen atom as its side chain—Glycine doesn't introduce steric hindrance that would otherwise limit the movement of protein segments.

Essence of Glycine in Flexibility

Because Glycine allows for tight turns and less restricted movement in certain regions, it's instrumental in places like loops or hinges where larger amino acids might restrict motion. This characteristic is so significant that mutations involving the replacement of Glycine often result in proteins that are rigid, misfolded, or functionally compromised. Therefore, the role of Glycine in maintaining the perfect balance of structure and flexibility cannot be overstated in the realm of protein functionality.

Evolution has fine-tuned proteins to their current sophisticated levels of function and specificity. Over time, certain amino acids within proteins have been more likely to change than others, influenced by selective pressures according to the protein's roles and needs. This process is known as 'protein evolution.'

Despite the vast potential for variation, Glycine's presence remains steadfast in many proteins across various organisms due to its irreplaceable structural contribution.

Glycine: A Keystone in Evolutionary Design

The conservation of Glycine is a testament to its evolutionary importance. Proteins are not static; they adapt and evolve. But when a single Glycine residue holds a key structural or functional role in a protein, altering it could disrupt the delicate balance required for proper protein function. As a result, such critical Glycine residues become 'evolutionarily conserved,' staying unaltered through vast stretches of time, bearing witness to Glycine’s essential role in maintaining the intricate machinery of life at the molecular level.