Haemoglobin and Protein Structure Notes

Learning Objectives & Introduction

• Overview of key concepts in biochemistry relating to haemoglobin, its structure, and the underlying mechanisms that govern its function. This includes the relationship between protein structure and function, as well as the dynamics of oxygen and carbon dioxide transport.

Protein Structure-Function Relationship

Proteins as Biological Molecules

• Proteins are essential macromolecules that serve as the building blocks of organisms, playing critical roles in controlling biological reactions, cellular structure, and signaling.

• Examples include:

  • Enzymes: Catalysts that speed up biochemical reactions by lowering activation energy.

  • Hormones: Regulatory molecules that influence physiological processes such as growth, metabolism, and reproduction.

  • Haemoglobin: A protein in red blood cells responsible for oxygen transport and CO2 removal from tissues.

Structure of Amino Acids

• Composed of:

  • Amino group (NH2): Basic part of the amino acid that can accept protons.

  • Carboxyl group (COOH): Acidic part that can donate protons and is involved in forming peptide bonds.

  • R-group (side chain): Variable group that determines the unique characteristics (polarity, charge, size) of each amino acid.

  • The arrangement of amino acids in a polypeptide chain is critical for determining the protein's three-dimensional structure and function.

• Four groups based on R-group chemistry:

  • Hydrophobic: Nonpolar side chains that typically avoid water, playing a role in protein folding.

  • Hydrophilic: Polar side chains that interact favorably with water, often found on the surface of proteins.

  • Positively charged (basic): Side chains that can accept protons and are typically found in the active sites of enzymes.

  • Negatively charged (acidic): Side chains that can donate protons, also found in enzyme active sites and metal-binding sites.

Peptide Bond Formation

• Proteins are polymers of amino acids, linked by peptide bonds. These bonds are formed through a condensation reaction, which results in the release of a water molecule as the carboxyl group of one amino acid reacts with the amino group of another amino acid.

• Dipeptides (two amino acids) and polypeptides (multiple amino acids) are synthesized sequentially based on genetic coding.

Levels of Protein Structure

Primary Structure

• The primary structure of a protein is the sequence of amino acids in the polypeptide chain, which is defined by the nucleotide sequence in genes. Each protein has a unique primary sequence that determines its properties and function.

• Example: The primary structure of human haemoglobin includes specific sequences of amino acids that are critical for its proper function in oxygen binding and release.

Secondary Structure

• The secondary structure refers to local folded structures that form within a polypeptide due to hydrogen bonding between the backbone atoms. Common secondary structures include:

  • Alpha-helix: A right-handed coil that is stabilized by hydrogen bonds between every fourth amino acid.

  • Beta-pleated sheet: Formed by hydrogen bonds between two or more polypeptide chains that are parallel or antiparallel, providing strength and stability.

Tertiary Structure

• The tertiary structure refers to the overall three-dimensional shape of a single polypeptide chain, which is maintained by multiple types of interactions, including:

  • Hydrogen bonds: Interactions between polar side chains.

  • Ionic bonds: Electrostatic attractions between positively and negatively charged side chains.

  • Hydrophobic interactions: Nonpolar side chains clustering away from water, aiding in the overall protein folding.

  • Van der Waals interactions: Weak attractions between closely positioned atoms.

  • Disulfide bridges: Covalent bonds formed between cysteine residues, adding stability to the protein's structure.

Quaternary Structure

• The quaternary structure applies to proteins formed by multiple polypeptide chains (subunits).

• An example is haemoglobin, which consists of four polypeptide subunits (two alpha and two beta chains), each with an associated haem group responsible for oxygen binding. This arrangement allows for cooperative binding, enhancing the protein's efficiency in oxygen transport.

Denaturation of Proteins

• Causes of denaturation include exposure to extreme heat, ultraviolet radiation, organic solvents, or strong acids/bases, which disrupt the weak bonds maintaining the protein structure.

• Denaturation alters the protein's shape, leading to loss of function, which can have critical biological implications, such as enzyme inactivation.

Physiological pH

Definition and Importance

• Physiological pH refers to the concentration of H+ in a solution, typically maintained between 7.35 and 7.45, essential for optimal enzyme activity and biochemical reactions in the body. Deviations from this range can lead to physiological disturbances.

• Acidosis: Defined as a condition where blood pH falls below 7.35, leading to increased acidity that can disrupt cellular processes.

• Alkalosis: A condition where blood pH rises above 7.45, resulting in excess alkalinity, which can affect nerve and muscle function.

Sources of Acids in the Body

• Acids in the body arise from normal metabolic processes, particularly during the metabolism of carbohydrates, proteins, and fats.

• CO2, produced during cellular respiration, acts like an acid as it reacts with water to form carbonic acid in solution.

• Fixed acids (e.g., from the metabolism of amino acids) also contribute to the overall acid-base balance in the body.

Buffering Systems

• The body relies on various buffering systems to maintain stable pH levels. The primary buffers in the blood include:

  • Bicarbonate (HCO3-): A crucial component of the buffer system that neutralizes excess acids.

  • Proteins: Including haemoglobin, which can bind to excess H+ ions, aiding in pH maintenance.

Blood Transportation of O2 & CO2

Oxygen Transport

• Approximately 98.5% of oxygen in the blood is bound to haemoglobin within red blood cells; the remaining 1.5% is dissolved in plasma.

• Cooperative binding allows hemoglobin to bind up to four O2 molecules, enhancing its efficiency in oxygen delivery to tissues where it is needed.

Oxygen Saturation Curve

• The sigmoidal shape of the oxygen saturation curve indicates cooperative binding. The affinity of haemoglobin for oxygen changes with varying partial pressure of O2, facilitating the loading and unloading of oxygen under different physiological conditions.

• The Bohr effect describes how increased levels of CO2 and H+ (lower pH) enhance oxygen delivery to tissues by decreasing haemoglobin's affinity for oxygen.

Carbon Dioxide Transport

• Around 60% of CO2 transported in the blood exists as bicarbonate ions (HCO3-), formed when CO2 reacts with water in cells.

• The remaining CO2 is carried:

  • As carbamino compounds (bound to proteins),

  • Dissolved in plasma (a small percentage).

• The Haldane effect states that deoxygenated blood can carry more CO2, facilitating CO2 removal from tissues and ensuring effective gas exchange in the lungs.

Cyanosis

Definition

• Cyanosis is a physical sign characterized by a bluish discoloration of the skin and mucous membranes due to a lack of oxygen in the blood.

• Central cyanosis: Typically indicates systemic hypoxia affecting the torso at low arterial O2 saturation.

• Peripheral cyanosis: Affects the extremities, such as fingers and toes, often stemming from reduced blood flow or localized hypoxemia.

Causes

• Cyanosis can result from a variety of conditions, including respiratory diseases (e.g., chronic obstructive pulmonary disease), congenital heart defects impeding normal blood oxygenation, and metabolic disorders that interfere with oxygen delivery.

• Exposure to carbon monoxide can increase levels of deoxyhaemoglobin, leading to cyanosis even in the presence of adequate oxygen levels.

Summary of Effects on Hb

• Carbon monoxide binds to haemoglobin with much greater affinity than oxygen, forming carboxyhemoglobin and leading to impaired oxygen transport, which can induce serious tissue hypoxia and illness.

The oxygen saturation curve, also known as the oxygen dissociation curve, illustrates the relationship between the partial pressure of oxygen (pO2) and the percentage of haemoglobin saturated with oxygen. This curve is typically sigmoidal (S-shaped) rather than linear, reflecting the cooperative binding nature of haemoglobin. As oxygen binds to the haemoglobin molecules, it enhances the binding of additional oxygen molecules—a phenomenon known as cooperative binding.

Key features of the oxygen dissociation curve include:

• High Affinity in Lungs: Under higher pO2 conditions (such as in the lungs), haemoglobin binds to oxygen more readily, leading to higher saturation levels.

• Lower Affinity in Tissues: As pO2 decreases (as in active tissues), the affinity of haemoglobin for oxygen decreases, facilitating oxygen release where it is most needed.

• Bohr Effect: The curve shifts to the right with increased levels of carbon dioxide (CO2) and H+ ions (lower pH). This means that in conditions of higher metabolic activity (which produce CO2 and decrease pH), haemoglobin releases oxygen more easily, enhancing oxygen delivery to tissues. Conversely, the curve shifts to the left with lower CO2 and H+, indicating increased binding affinity for oxygen.

• Physiological Implications: The curve's shape enables efficient oxygen uptake in the lungs and oxygen delivery in peripheral tissues, adjusting to varying metabolic demands under different physiological conditions.