Proteins & Amino Acids: Structures, Digestion, and Absorption

Introduction

This study guide refers to the material presented in the FSHN 350 video, alongside specific readings from Gropper, Chapters 6 (pp. 187-197; 216-232).

Why Are Proteins Amazing?

Proteins display an extraordinary level of diversity, which allows them to perform numerous functions within living organisms.

Functions of Proteins

Proteins serve several critical functions in the body, including:

• Structure: Proteins provide the structural framework for cells and tissues.

• Catalysis: Enzymatic proteins catalyze biochemical reactions.

• Movement: Proteins are involved in muscular contraction and movement.

• Transport: Proteins transport molecules across cell membranes and within the bloodstream.

• Communication: Proteins function as signaling molecules to convey information between cells.

• Protection: Proteins such as antibodies play a key role in the immune response.

• Regulation of Fluid Balance: Proteins help maintain osmotic balance in fluids.

• Regulation of pH: Proteins assist in maintaining the acid-base balance of the body.

Importantly, intact dietary proteins are not utilized directly; instead, their constituent amino acids are used to synthesize body proteins.

Proteins and Body Functions

Proteins are pivotal in maintaining various bodily functions:

• Energy: Proteins provide 4 kcal/g when metabolized, serving as energy sources alongside lean body mass (LBM).

• Essential Nitrogen Source: Proteins supply nitrogen, which is vital for numerous physiological processes.

• Hormones & Enzymes: Many hormones and enzymes are proteins that regulate metabolism and physiological responses.

• Fluid Balance: They maintain proper fluid levels in tissues and cells.

• Transport in Cell Membranes and Blood: They facilitate the movement of substances across cellular and vascular barriers.

• Blood Clotting: Certain proteins are involved in the cascade of reactions that lead to clot formation.

• Acid-Base Balance: Proteins help to buffer changes in pH levels in body fluids.

• Cell Repair: Proteins play a role in the repair and regeneration of tissues.

• Structural Proteins: Include fibrous proteins, muscle proteins (contractile), and immunoglobulins/antibodies for immune defense, as well as proteins involved in vision.

Regulation of Fluid Balance by Albumin

Albumin regulates fluid movement in the interstitial spaces, preventing edema. The processes involved include:

1. Blood Circulation: Blood is pumped from the heart into blood vessels, and the narrow diameter of capillary beds leads to increased blood pressure.

2. Fluid Movement: This pressure forces fluids and nutrients (excluding albumin) out of capillaries into the interstitial space.

3. Albumin's Role: Increasing albumin concentration draws fluids back into capillaries. When albumin levels are low, edema can occur, often seen in protein deficiency.

Macronutrient Composition: Differences Between Proteins, Carbohydrates, and Lipids

• Carbohydrates: Composed of monosaccharides such as glucose.

• Lipids: Made up of fatty acids and triglycerides.

• Proteins: Composed of amino acids, which are distinguished by their unique properties (side chains).

Structures of Proteins

The structure of proteins is critical to function, categorized into four levels:

Primary Structure

• Definition: The primary structure is the specific sequence of amino acids in a polypeptide chain, which is defined by the genetic code (DNA).

• Key Amino Acids:

  • Valine (Val)

  • Aspartate (Asp)

  • Lysine (Lys)

  • Serine (Ser)

  • Tyrosine (Tyr)

  • Phenylalanine (Phe)

• Importance: The sequence dictates the protein's identity and function.

Secondary Structure

• Types:

  • α-Helix: A cylindrical structure formed by the coiling of the polypeptide chain, stabilized by hydrogen bonds between carboxylic acid and amino groups.

  • β-Pleated Sheet: Formed when the polypeptide chain is extended, resulting in side chains that may fold upon themselves.

  • Random Coil: An unstable structure owing to certain amino acids' side chains that interfere with each other.

Tertiary Structure

• Definition: The three-dimensional structure of a protein, determined by interactions between side chains, including covalent and various non-covalent interactions.

• Example: The tertiary structure of a-lactalbumin illustrates the complexity of interactions that lead to a functional protein shape.

Quaternary Structure

• Definition: Formed when two or more polypeptides interact to form a larger functional protein.

• Example: Hemoglobin, which consists of four polypeptide chains, binds to oxygen; these subunits exhibit cooperative behavior to enhance oxygen transport.

• Interactions: Quaternary structures are held together by hydrogen bonds and electrostatic interactions called salt bridges.

Gene-Protein Connection

The relationship between genes and protein synthesis:

1. Gene Expression: A single gene directs the synthesis of a single polypeptide, while it is possible for multiple genes to contribute to individual protein synthesis.

2. Protein Synthesis Steps:

   • Cellular Signals: Stimulate the need for protein synthesis.

   • Transcription: DNA is transcribed to produce mRNA.

   • Translation: mRNA is translated by ribosomes to synthesize proteins, guided by codons correlating to specific amino acids and their respective tRNA.

Amino Acids at Physiological pH

• Each amino acid consists of a central carbon (C), an amino group (–NH3+), a carboxyl group (–COO−), and a unique side chain (R) that differentiates each amino acid.

• In aqueous solutions at physiological pH, amino acids predominantly exist as zwitterions, meaning they carry both positive and negative charges simultaneously.

Ionization States of Amino Acids

• The ionization state of amino acids changes with pH levels.

• At near physiological pH:

  • Zwitterion Form: Predominates with no net charge; the amino group is protonated, while the carboxyl group is deprotonated.

Classification of Amino Acids

Amino acids can be classified based on:

1. Structure of the Side Chain

2. Net Electrical Charge

3. Polarity (determined by side chains)

4. Essentiality: There are 20 amino acids, classified as essential (required in diet), nonessential (can be synthesized by the body), and conditionally essential (needed at certain life stages).

   • Essential Amino Acids (9) include:

     • Phenylalanine

     • Valine

     • Tryptophan

     • Threonine

     • Isoleucine

     • Methionine

     • Histidine

     • Leucine

     • Lysine

   • Conditionally Essential Amino Acids include:

     • Tyrosine (from Phenylalanine)

     • Cysteine (from Methionine and Serine)

     • Proline (from Glutamate)

     • Arginine (from Glutamine and Aspartate)

     • Glutamine (from Glutamate and Ammonia)

Essential Amino Acid Structures

A visual resource to learn structures can be found here: https://quizlet.com/onejames/folders/amino-acids?i=zwgs&x=1xqt

Aliphatic Amino Acids

• Definition: Aliphatic amino acids contain side chains that are open-chain carbon structures and are generally non-polar and hydrophobic. Hydrophobicity increases with the number of carbon atoms.

• Characteristics:

  • Amino acids like alanine and glycine can be ambivalent, located within or outside protein molecules.

  • Examples include:

  • Glycine (Gly)

  • Alanine (Ala)

  • Valine (Val)

  • Leucine (Leu)

  • Isoleucine (Ile)

Branched-Chain Amino Acids (BCAAs)

• BCAAs, especially leucine, play a significant role in stimulating protein synthesis and regulatory processes in muscle cells post-exercise.

• They are efficiently absorbed by skeletal muscle, heart, kidneys, and adipose tissues, where they contribute to muscle protein synthesis or oxidative processes during fasting.

Hydroxylic/Uncharged/Polar Amino Acids

This category includes amino acids that carry hydroxyl groups but remain uncharged at physiological pH. They include:

• Examples:

  • Serine (Ser)

  • Threonine (Thr)

  • Asparagine (Asn)

Charged Polar Amino Acids

Certain amino acids possess charged side chains at physiological pH:

• Examples:

  • Aspartic Acid (Asp)

  • Glutamic Acid (Glu)

  • Lysine (Lys)

  • Arginine (Arg)

  • Histidine (His)

Aromatic Amino Acids

Amino acids that possess an aromatic ring feature include:

• Examples:

  • Tyrosine (Tyr)

  • Phenylalanine (Phe)

  • Tryptophan (Trp)

Peptide Formation

• Peptide Bond: The bond formed between the carboxyl group of one amino acid and the amine group of the next through dehydration synthesis, resulting in a dipeptide, tripeptide, or polypeptide.

• N-terminus: The beginning of the amino acid sequence, where an amine group is present.

• C-terminus: The end of the sequence, capped with a carboxyl group.

• Peptide chains can be described starting at the N-terminus and following sequentially to the C-terminus. A tetra-peptide example would be referred to as "alanyl-tyrosyl-aspartyl-glycine."

Conclusion

Understanding the structures and functions of proteins and amino acids is crucial for comprehending broader biological processes, including enzyme action, metabolic pathways, and overall physiological regulation in organisms. By engaging with these concepts, one gains insight into the molecular basis of life and the critical role these biomolecules play in maintaining health and function.