Proteins Are Made Of Smaller Subunits Called

Proteins Are Made of Smaller Subunits Called Amino Acids: A Deep Dive

Proteins are the workhorses of the cell, performing a vast array of functions crucial for life. From catalyzing biochemical reactions as enzymes to providing structural support as components of the cytoskeleton, their roles are multifaceted and indispensable. But what exactly are proteins, and what are these smaller subunits that make them up? The answer is amino acids. This article will delve into the fascinating world of amino acids, their structure, their properties, and how they combine to form the incredible diversity of proteins found in all living organisms.

Understanding Amino Acids: The Building Blocks of Proteins

Amino acids are organic molecules containing a central carbon atom (also known as the alpha carbon) bonded to four different chemical groups:

• An amino group (-NH2): This group is basic and readily accepts a proton (H+).
• A carboxyl group (-COOH): This group is acidic and readily donates a proton (H+).
• A hydrogen atom (-H): A simple hydrogen atom.
• A variable side chain (R group): This is the unique part of each amino acid, giving it distinct chemical properties.

It's this R group that distinguishes the 20 standard amino acids used by cells to build proteins. These R groups can be:

• Nonpolar (hydrophobic): These side chains are repelled by water and tend to cluster together in the interior of proteins. Examples include alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, and proline. Proline is unique as its R group forms a ring structure with the amino group, restricting its flexibility.

• Polar (hydrophilic): These side chains are attracted to water and often found on the surface of proteins, interacting with the aqueous environment. Examples include serine, threonine, cysteine, tyrosine, asparagine, and glutamine. Cysteine is noteworthy for its ability to form disulfide bonds with other cysteine residues, contributing to protein structure.

• Charged (hydrophilic): These side chains carry a net positive or negative charge at physiological pH. Positively charged amino acids include lysine, arginine, and histidine. Negatively charged amino acids include aspartic acid and glutamic acid. The charged nature of these side chains contributes significantly to protein-protein interactions and overall protein function.

The Peptide Bond: Linking Amino Acids Together

Amino acids link together to form proteins through a process called peptide bond formation. This is a dehydration reaction, where the carboxyl group of one amino acid reacts with the amino group of another, releasing a water molecule and forming a peptide bond, a covalent bond between the carbon atom of the carboxyl group and the nitrogen atom of the amino group. This bond is relatively strong and contributes significantly to the stability of the protein structure.

A chain of amino acids linked by peptide bonds is called a polypeptide. Proteins can consist of one or more polypeptide chains. The sequence of amino acids in a polypeptide chain is its primary structure, and this sequence is dictated by the genetic code. The genetic code, residing within DNA, is transcribed into messenger RNA (mRNA), which is then translated by ribosomes to produce the specific amino acid sequence of a protein.

Protein Structure: Beyond the Primary Sequence

The primary structure of a protein, while crucial, doesn't tell the whole story. The protein’s final three-dimensional structure dictates its function. These higher levels of structure are:

Secondary Structure:

This refers to local folding patterns within the polypeptide chain, stabilized by hydrogen bonds between the backbone atoms (the amino and carboxyl groups). Common secondary structure elements include:

• α-helices: A right-handed coiled structure, stabilized by hydrogen bonds between every fourth amino acid.
• β-sheets: Extended, pleated structures formed by hydrogen bonds between adjacent polypeptide chains or segments of the same chain. β-sheets can be parallel (chains run in the same direction) or antiparallel (chains run in opposite directions).

Tertiary Structure:

This refers to the overall three-dimensional arrangement of a polypeptide chain, including the spatial relationships between secondary structure elements. It is stabilized by a variety of interactions including:

• Disulfide bonds: Covalent bonds between cysteine residues.
• Hydrogen bonds: Between various polar side chains.
• Ionic bonds (salt bridges): Between oppositely charged side chains.
• Hydrophobic interactions: Clustering of nonpolar side chains within the protein core.

Quaternary Structure:

This refers to the arrangement of multiple polypeptide chains in a protein complex. Many proteins consist of two or more polypeptide chains (subunits), each with its own tertiary structure, that associate to form a functional protein. Hemoglobin, for example, is a tetramer composed of four polypeptide subunits.

The Importance of Amino Acid Sequence and Protein Folding

The precise sequence of amino acids in a protein is absolutely crucial for its function. A single amino acid change can dramatically alter a protein's structure and activity. This is evident in diseases like sickle cell anemia, where a single amino acid substitution in the hemoglobin protein leads to a change in its shape and function, resulting in abnormal red blood cells.

The process of protein folding, whereby a polypeptide chain adopts its functional three-dimensional structure, is a complex and intricate process. The interactions between amino acid side chains guide the folding process, but the precise mechanisms are still being actively researched. Improper folding can lead to misfolded proteins that are non-functional or even harmful, accumulating in the cell and contributing to diseases such as Alzheimer's and Parkinson's.

Beyond the 20 Standard Amino Acids

While cells use 20 standard amino acids to build proteins, there are other amino acids that can be found in proteins after they are synthesized. These modifications often occur through post-translational modifications, which are chemical alterations made to a protein after it has been translated from mRNA. These modifications can affect protein function, stability, and localization. Examples include phosphorylation (addition of a phosphate group), glycosylation (addition of a carbohydrate group), and acetylation (addition of an acetyl group).

The Role of Proteins in Biological Systems: A Diverse Cast of Characters

Given their diverse structures and properties, proteins participate in a vast array of essential biological processes. Examples include:

• Enzymes: Catalyze biochemical reactions.
• Structural proteins: Provide support and shape to cells and tissues (e.g., collagen, keratin).
• Transport proteins: Carry molecules across cell membranes (e.g., ion channels, membrane transporters).
• Motor proteins: Generate movement (e.g., myosin, kinesin).
• Hormones: Act as chemical messengers (e.g., insulin, growth hormone).
• Antibodies: Part of the immune system, binding to and neutralizing foreign substances.
• Receptor proteins: Bind to signaling molecules and initiate cellular responses.
• Storage proteins: Store amino acids (e.g., casein in milk).

Conclusion: A World of Protein Diversity and Function

The incredible diversity of proteins stems from the combinatorial possibilities of the 20 standard amino acids and the intricate folding patterns they adopt. Understanding the structure and function of proteins is essential for comprehending the complexities of life itself. From the simple act of peptide bond formation to the sophisticated mechanisms of protein folding and post-translational modifications, the world of proteins is a dynamic and fascinating realm that continues to be explored and unveiled by scientists around the globe. Further research into protein structure, function, and misfolding holds the potential to yield critical insights into diseases and to develop novel therapeutic strategies. The study of amino acids and proteins, therefore, remains a central pillar in biological research.