BIO 2101 AM 09-20-2023

Amino Acids and Proteins

Carboxyl Group:

• The carboxyl group (–COOH) is essential in identifying the structure of amino acids, which are the building blocks of proteins. This functional group contributes to the acidic properties of the amino acids by donating protons in solution.

Basic Structure of Amino Acids:

• Each amino acid has a basic structure comprising a central carbon atom (the alpha carbon) bonded to:

  • An amino group (–NH2), which is basic and can accept protons.

  • A carboxyl group (–COOH), which is acidic and can donate protons.

  • A hydrogen atom.

  • A variable R group, also known as the side chain, which is unique to each amino acid and defines its characteristics, such as hydrophobicity or hydrophilicity.

• There are 20 standard amino acids, which are categorized into essential amino acids (not synthesized by the body and must be ingested) and non-essential amino acids (which can be synthesized by the body). The specific sequence of these amino acids is what determines the protein's function.

Peptide Bond Formation:

• Amino acids link together through a dehydration synthesis reaction, releasing a water molecule as a byproduct. This process forms a peptide bond, a covalent bond between the carbon atom of the carboxyl group of one amino acid and the nitrogen atom of the amino group of another, resulting in a dipeptide. This process continues to form polypeptides.

Structure of Proteins

Polypeptides:

• Polypeptides are long chains of amino acids, which can range from as few as 50 to over a thousand amino acids in length. The sequence in which these amino acids are arranged dictates the polypeptide's structure and function, leading to the diverse range of proteins in organisms.

Levels of Protein Structure:

• Primary Structure: The linear sequence of amino acids in a polypeptide chain is determined by the genetic code. Any change in this sequence can lead to variations in protein structure and function.

• Secondary Structure: The polypeptide chain begins to fold into specific structures, predominantly influencing the helices and sheets formed when hydrogen bonds stabilize between nearby amino acids. The most common secondary structures are alpha helices and beta-pleated sheets.

• Tertiary Structure: This represents the 3D folding pattern due to interactions among various side chains (R groups) of the amino acids, including hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges. This folding is critical for the protein's biological function.

• Quaternary Structure: Some proteins consist of multiple polypeptide chains, known as subunits, that come together to form a functional protein. Hemoglobin, for example, has a quaternary structure consisting of four subunits (two alpha and two beta chains) arranged in a specific way to efficiently transport oxygen.

Hemoglobin Structure

• Hemoglobin: A tetrameric protein indispensable for oxygen transport in vertebrates. Each of the four polypeptide chains contains a heme group, which is an iron-containing compound that reversibly binds to oxygen molecules. The binding and release of oxygen are facilitated by changes in the conformation of the hemoglobin molecule, allowing efficient delivery of oxygen to tissues and organs.

Sickle Cell Anemia:

• This genetic disorder results from a point mutation in the beta-globin gene, leading to a single amino acid substitution (valine replacing glutamic acid) in the beta chains of hemoglobin. The altered hemoglobin (hemoglobin S) polymerizes under low oxygen conditions, forming rigid structures that distort red blood cells into a sickle shape. This deformation not only reduces the cell's ability to transport oxygen but also leads to blockages in blood vessels, causing pain and other complications. Interestingly, carriers of the sickle cell trait have a selective advantage against malaria, explaining why the allele persists in regions where malaria is endemic.

Kidney Function and Nephrons

• The nephron is the fundamental structural and functional unit of the kidney, critical for maintaining fluid and electrolyte balance. It comprises two main components:

  • Renal Corpuscle: Comprises the glomerulus (a tuft of capillaries) and Bowman's capsule, where the filtration process initiates.

  • Renal Tubules: Made up of segments (proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct) that reabsorb vital substances and regulate water and electrolyte balance.

Filtration Process:

• The glomerulus functions primarily through passive diffusion. Plasma is filtered into Bowman's capsule while cells and large proteins are retained in the bloodstream, resulting in the formation of filtrate.

Selective Reabsorption:

• As filtrate traverses through the renal tubules, essential substances like glucose, various ions, and amino acids are reabsorbed back into the bloodstream through active and passive transport mechanisms, while waste products (e.g., urea) are left for excretion.

Control of Water Reabsorption:

• Hormones play a crucial role in regulating water and electrolyte balance in the body:

  • Vasopressin (ADH): Increases the permeability of the collecting ducts to water, enhancing water reabsorption, particularly crucial during dehydration.

  • Aldosterone: A steroid hormone that regulates sodium reabsorption in the distal convoluted tubule, indirectly influencing water retention by affecting blood volume and pressure.

Gas Exchange in Lungs

• Alveoli: Tiny air sacs in the lungs where gas exchange occurs comprised of simple squamous epithelium, allowing efficient diffusion of oxygen and carbon dioxide through their thin walls. Oxygen enters the bloodstream, while carbon dioxide is expelled from the blood into the alveoli to be exhaled. The high surface area and rich capillary network surrounding the alveoli enhance gas exchange efficiency.

• Altitude Effects: At higher altitudes, the reduced atmospheric pressure lowers oxygen availability, affecting the efficiency of gas exchange. Understanding how pressure differences influence oxygen diffusion is vital, particularly for those living in or traveling to high-altitude locations.

Exam Preparation and Practical Tips

• Structure and Function: It’s imperative to understand how the structure of biological molecules relates to their function, as this principle underlines many concepts in biology.

• Specific Terminology: Employ precise scientific terms when discussing anatomical structures and functions, such as using "erythrocyte" to refer to red blood cells.

• Practical Skills: Familiarization with histological slides and terminology is crucial for practical examinations and ensures comprehension of tissue structures and functions.

• Hydration and Kidney Function: Acknowledge how hydration levels critically impact kidney filtration and reabsorption processes, affecting overall homeostasis.

• Make a note of the exam rescheduling; the first exam is now on October 2.