Which of the following is an example of a protein found in cells?

Enzymes are a type of protein that serve as biological catalysts in cells, facilitating and speeding up biochemical reactions. They are composed of long chains of amino acids that fold into specific three-dimensional structures, allowing them to interact with substrates in a highly specific manner. This structural characteristic is crucial for their function, as the unique shape of an enzyme's active site determines its specificity for particular substrates.

In cellular metabolism, enzymes play vital roles in processes such as digestion, DNA replication, and energy production, making them essential for maintaining life. Their crucial functions highlight their importance in the biochemistry of living organisms, allowing for the regulation and control of complex biochemical pathways.

The other choices do not represent proteins. Glucose is a carbohydrate, ATP is a nucleotide involved in energy transfer, and fatty acids are components of lipids. Thus, it is clear that enzymes are the appropriate example of a protein found in cells, showcasing the critical role proteins play in biological processes.