Proteins: Structure and Functions

Proteins are among the most abundant organic molecules in living systems and exhibit the widest variety of functions among macromolecules.
Functions of proteins include structural, regulatory, transport, storage, protective, enzymatic, and toxic roles.
Each cell may contain thousands of distinct proteins, each serving a unique function based on its specific amino acid sequence.
Proteins are polymers made up of amino acids organized in a linear sequence, exhibiting diverse shapes and molecular weights.
Globular proteins (e.g., hemoglobin) are typically more rounded, while fibrous proteins (e.g., collagen in skin) have elongated structures.
Protein shape is crucial for its function; many types of chemical bonds help maintain this shape.
Changes in temperature, pH, or chemicals can permanently alter a protein’s shape, resulting in loss of function (denaturation).
All proteins are composed of arrangements of 20 standard amino acids, plus two rare amino acids: selenocysteine and pyrrolysine.
Hormones, often small proteins or steroids secreted by endocrine cells, play key roles in regulating physiological processes (e.g., insulin regulates blood glucose levels).
Enzymes are complex or conjugated proteins produced by living cells that act as catalysts in biochemical reactions, such as digestion.
Each enzyme is substrate-specific, facilitating breakdown (catabolic enzymes) or synthesis (anabolic enzymes) processes.

Amino acids are the monomers that make up proteins, each containing a central carbon (α carbon), an amino group (NH2), a carboxyl group (COOH), a hydrogen atom, and a unique R group.
The term "amino acid" reflects the presence of both an amino and a carboxyl group in their structure.
There are 20 common amino acids, with nine being essential for humans, as they cannot be synthesized by the body and must be obtained through diet.
The R group determines the characteristics of each amino acid, categorizing them as acidic, basic, polar, or nonpolar.
Nonpolar amino acids (e.g., valine, methionine, alanine) are hydrophobic, while polar amino acids (e.g., serine, threonine, cysteine) are hydrophilic.
Lysine and arginine possess positively charged side chains, classifying them as basic amino acids.
Proline is atypical, as its R group forms a ring structure linked to the amino group, differing from standard amino acid composition.