Acid-Base Biochemistry Dr. Catherine Street Consultant Clinical Biochemist Colchester Hospital university NHS Foundation Trust
Acid-Base Biochemistry Definitions Methods Physiology Pathology
Acid-Base BiochemistryDefinitions What is an acid? What is a base?
Acid-Base BiochemistryDefinitions Definitions of an acid Taste Boyle Arrhenius Bronsted-Lowry Lewis
Acid-Base BiochemistryDefinitions Taste Acere – tasting sour Lemon juice Vinegar Definition - Thousands of years old
Acid-Base BiochemistryDefinitions Robert Boyle 17th century Acids taste sour, are corrosive to metals, change litmus (a dye extracted from lichens) red, and become less acidic when mixed with bases (Alkali). Bases (Alkali) feel slippery, change litmus blue, and become less basic (alkaline) when mixed with acids.
Acid-Base BiochemistryDefinitions Arrhenius Arrhenius suggested that acids are compounds that contain hydrogen and can dissolve in water to release hydrogen ions into solution. For example, hydrochloric acid (HCl) dissolves in water as follows: H2O HCl (g)→ H+ (aq) + Cl-(aq)
Acid-Base BiochemistryDefinitions Arrhenius defined bases as substances that dissolve in water to release hydroxide ions (OH-) into solution. For example, a typical base according to the Arrhenius definition is sodium hydroxide (NaOH): H2O NaOH (s)→ Na+ (aq) + OH-(aq)
Acid-Base BiochemistryDefinitions The Arrhenius definition of acids and bases explains a number of things. Arrhenius's theory explains why all acids have similar properties to each other (and, conversely, why all bases are similar): because all acids release H+ into solution (and all bases release OH-).
Acid-Base BiochemistryDefinitions The Arrhenius definition also explains Boyle's observation that acids and bases counteract each other. This idea, that a base can make an acid weaker, and vice versa, is called neutralization.
Acid-Base BiochemistryDefinitions Neutralization: As you can see from the equations, acids release H+ into solution and bases release OH-. If we were to mix an acid and base together, the H+ion would combine with the OH- ion to make the molecule H2O, or plain water: H+ (aq) + OH-(aq) → H2O
Acid-Base BiochemistryDefinitions The neutralization reaction of an acid with a base will always produce water and a salt, as shown below: Acid Base Water Salt HCl + NaOH → H2O + NaCl HBr + KOH → H2O + KBr
Acid-Base BiochemistryDefinitions Limitations of Arrhenius The Arrhenius definition does not explain why some substances, such as common baking soda (NaHCO3), can act like a base even though they do not contain hydroxide ions.
Acid-Base BiochemistryDefinitions Brǿnsted-Lowry 1923 An acid is any chemical species that donates a proton to another chemical species (proton donor) A base is any chemical species that accepts a proton from another chemical species (Proton acceptor)
Acid-Base BiochemistryDefinitions The Brønsted-Lowry definition of acids is very similar to the Arrhenius definition, any substance that can donate a hydrogen ion is an acid (under the Brønsted definition, acids are often referred to as proton donors because an H+ ion, hydrogen minus its electron, is simply a proton).
Acid-Base BiochemistryDefinitions The Brønsted definition of bases is, however, quite different from the Arrhenius definition. Arrhenius base releases hydroxyl ions whereas the Brønsted base is defined as any substance that can accept a hydrogen ion.
Acid-Base BiochemistryDefinitions The Brønsted-Lowry definition includes the Arrhenius bases so NaOH and KOH, as we saw above, would still be considered bases because they can accept an H+ from an acid to form water. But it extends the concept of a base and introduces the concept of conjugate acid-base pairs
Acid-Base BiochemistryDefinitions The removal of a proton (hydrogen ion) from an acid produces its conjugate base, which is the acid with a hydrogen ion removed, and the reception of a proton by a base produces its conjugate acid, which is the base with a hydrogen ion added
Acid-Base BiochemistryDefinitions The Brønsted-Lowry definition also explains why substances that do not contain OH- ions can act like bases. Baking soda (NaHCO3), for example, acts like a base by accepting a hydrogen ion from an acid as illustrated below: Acid Base Salt HCl + NaHCO3→ H2CO3 + NaCl
Acid-Base BiochemistryDefinitions Lewis definition 1923 A substance that can accept an electron pair from a base; thus, AlCl3, BF3, and SO3 are acids. The Lewis theory defines an acid as a species that can accept an electron pair from another atom, and a base as a species that can donate an electron pair to complete the valence shell of another atom
Acid-Base BiochemistryDefinitions pHUnder the Brønsted-Lowry definition, both acids and bases are related to the concentration of hydrogen ions present. Acids increase the concentration of hydrogen ions, while bases decrease the concentration of hydrogen ions (by accepting them). The acidity or basicity of something therefore can be measured by its hydrogen ion concentration.
Acid-Base BiochemistryDefinitions In 1909, the Danish biochemist Sören Sörensen invented the pH scale for measuring acidity. The pH scale is described by the formula: pH = -log [H+] Note: concentration is commonly abbreviated by using square brackets, thus [H+] = hydrogen ion concentration. When measuring pH, [H+] is in units of moles of H+ per litre of solution.
Acid-Base BiochemistryMethods pH electrode
Acid-Base BiochemistryMethods pH electrode
Acid-Base BiochemistryMethods How the pH Electrode works • As the pH Glass comes into contact with an aqueous substance to measure, a gel layer forms on the membrane. This also happens on the inside of the glass layer. .
Acid-Base BiochemistryMethods How the pH Electrode works • The pH value of the aqueous solution will either force Hydrogen Ions out of the gel layer or into this layer. The Internal buffer in the glass electrode has a constant pH value and this keeps the potential at the inner surface of the membrane constant.
Acid-Base BiochemistryMethods How the pH Electrode works • The membrane potential is therefore the difference between the inner and outer charge. If you then factor in the stable potential of reference electrode, you have a voltage proportional to the pH value of the solution being measured, this being approximately 58mV/pH unit @ 20ºC
Acid-Base BiochemistryMethods Other methods you need to know and understand • Carbon dioxide electrode • Oxygen electrode • Laboratory measurement of bicarbonate • Ion selective electrodes for K+ Na+ Cl-
Acid-Base BiochemistryPhysiology What is Physiological pH range?
Acid-Base BiochemistryPhysiology Extracellular fluid pH 7.35 – 7.46 (35-45 nmol/L) Does this apply to whole body ?any different pH ranges elsewhere
Acid-Base BiochemistryPhysiology More extreme/variable pH range Digestive tract Gastric Juice 1.0-3.0 Pancreatic Juice 8.0-8.3 Intercellular organelles Lysosomal pH 4-5 Digestive and lysosomal enzymes function optimally at these pH ranges
Acid-Base BiochemistryPhysiology Traditionally use pH to measure acidity Problem 1. direction of pH change is opposite to increase/decrease of Hydrogen ion concentration 2. Use of log scale ‘masks’ the extent of the change -change of 0.3 in pH represents doubling/halving of hydrogen ion concentration
Acid-Base BiochemistryPhysiology • More recently – use Hydrogen ion concentration [H+] • Traditionalists and older equipment use pH • For large pH changes may not register change in units eg nmole/L to moles/L • Most practical - give both
Acid-Base BiochemistryPhysiology WHAT THE SOURCES OF ACID IN THE BODY?
Acid-Base BiochemistryPhysiology Sources of acid Metabolism of food Metabolism of drugs Inborn errors of metabolism
Acid-Base BiochemistryPhysiology Acid production from metabolism of food Sulphuric acid from metabolism of sulphur-containing amino acids of proteins Lactic acid from sugars Ketoacids from fats
Acid-Base BiochemistryPhysiology Acid production from metabolism of drugs Direct metabolism of drug to more acidic compound eg salicylates urates etc Induction of enzymes which metabolise other compounds (endogenous or exogenous) to acids
Acid-Base BiochemistryPhysiology Inborn errors of metabolism Organic acid disorders Lactic acidosis
Acid-Base BiochemistryPhysiology Greatest potential source of acid Carbon dioxide CO2 + H2O <=> H2CO3 (2) H2CO3 <=> H+ + HCO3- Potentially 15,000 mmol/24 hours
Acid-Base BiochemistryPhysiology Hydrogen ion homeostasis 1. buffering 2. excretion
Acid-Base BiochemistryPhysiology Buffering of hydrogen ions In health as hydrogen ions are produced they are buffered – limiting the rise in [H+]
Acid-Base BiochemistryPhysiology Buffer solutions consist of a weak acid and its conjugate base As hydrogen ions are added some will combine with the conjugate base and convert it to undissociated acid
Acid-Base BiochemistryPhysiology Bicarbonate – carbonic acid buffer system H+ + HCO3-<=> H2CO3 Addition of H+ drives reaction to the right Conversely Fall in H+ drives reaction to the left as carbonic acid dissociates producing more H+
Acid-Base BiochemistryPhysiology Buffering systems in blood Bicarbonate ions-most important Proteins including intracellular proteins Haemoglobin
Acid-Base BiochemistryPhysiology Buffer solutions operate most efficiently at [H+] that result in approximately equal concentration of undissociated acid and conjugate base But at normal extracellular fluid pH [H2CO3] 1.2 mmol whereas [HCO3-] is twenty times greater
Acid-Base Biochemistry Physiology The bicarbonate system is enhanced by the fact that carbonic acid can be formed from CO2 or disposed of by conversion to CO2 CO2 + H2O <=> H2CO3
Acid-Base BiochemistryPhysiology For every hydrogen ion buffered by bicarbonate – a bicarbonate ion is consumed. To maintain the capacity of the buffer system, the bicarbonate must be regenerated However, when bicarbonate is formed from carbonic acid (CO2 and H2O) equimolar amounts of [H+] are formed
Acid-Base BiochemistryPhysiology Bicarbonate formation can only continue if these hydrogen ions are removed This process occurs in the cells of the renal tubules where hydrogen ions are secreted into the urine and where bicarbonate is generated and retained in the body
Acid-Base BiochemistryPhysiology • 2 different processes • Bicarbonate regeneration (incorrectly reabsorption) • Hydrogen ion excretion
Acid-Base BiochemistryPhysiology Importance of Renal Bicarbonate Regeneration Bicarbonate is freely filtered through the glomerulus so plasma and glomerular filtrate have the same bicarbonate concentration At normal GFR approx 4300 mmol of bicarbonate would be filtered in 24 hr Without re-generation of bicarbonate the buffering capacity of the body would be depleted causing acidotic state In health virtually all the filtered bicarbonate is recovered