Clinical Chem Enzymology

Learning Outcome

To understand the clinically important enzymes and isoenzymes, as well as their changes in certain disorders.

Biological catalyst that increases the rate of a biochemical reaction
- catalyses the conversion of substrates → products
- specific
Not undergoing permanent changes or being consumed
Alter the rate, NOT the equilibrium point

Most are proteins or conjugated proteins
primary structure - linear sequence of the amino acids
secondary structure - three-dimensionally coiled polypeptides
tertiary structure - subsequent folding of secondary structure
Active site
- interaction and binding site for substrate
- a cleft or groove formed by the specific folding pattern of specific amino acid residues
- determines substrate specificity

change in pH affects the ionisation state of acidic or basic amino acids
- change the structure and shape of enzyme → affect/inactivate enzyme activity
each enzyme has an optimal pH value, ranging from pH 1.5-8.0

↑enzyme concentration
- ↑ active sites to convert substrate molecules into products
- needs to have an excess of substrate
zero-order kinetics
- a reaction under the condition of substrate excess
- all active sites of enzymes are saturated in an excess of substrate
- rate of reaction us independent of substrate concentration
- concentration of product formed over a specified period of time depends on the enzyme concentration

↑ substrate concentration, ↑ rate of product formation, at a constant enzyme concentration
rate of product formation is linear & proportional to the substrate concentration, until a limiting point is reached
limiting point
- all active sites are occupied with substrates
- rate of reaction at its maximum
- further increase in the substrate concentration does not increase the reaction rate

Name if the substrate/group targeted by the enzyme, followed by the suffix -ase
- e.g., urease → enzyme that hydrolyses urea
Name indicated the reaction catalysed by the enzyme
- e.g., glucose oxidase enzyme that catalyses the oxidation of glucose to gluconolactone & hydrogen peroxidase
Empirical name, e.g., trypsin & pepsin

^^Membrane-bound^^ glycoprotein on the outer layer of cell membrane
exists as a group of isoenzymes in different tissue sites
found on membranes and cell surfaces of small intestine, kidneys, liver, bone & placenta
majority (>80%) of alkaline phosphatase in serum is liver isoenzyme, followed by the bone isoenzyme & a small amount from intestine
Physiological functions:
- catalyses the hydrolysis of phosphate monoesters at alkaline pH
- releases inorganic phosphate from the substrate
- requires divalent cations as cofactors
- only found in the tissues under normal physiological condition
- increased production under specific condition may release it into circulation

significantly increased activity in obstructive hepatobiliary disease & osteoblast-mediated bone disease
- elevated serum alkaline phosphatase level
biliary tract obstruction: 3-10x the upper limit of reference interval
hepatobiliary disorders: alkaline phosphatase + other markers of hepatic function

primary or metastatic cancer with local bile duct obstruction & increasing leakage of liver isoenzyme

less common
hypophosphatasia, postmenopausal women receiving oestrogen therapy for osteoporosis, Wilson’s disease, magnesium deficiency, hypothyroidism & severe anaemia

An oxidoreductase important in anaerobic metabolic pathway
present in almost all tissue, but significant level in heart, skeletal muscle, liver & kidneys
higher concentration in the cytoplasm than serum
tetramers of either of the 2 types of subunits:
- muscle (M)
- heart (H)
Exhibits 5 isomeric forms:
- LDH1 - 4 heart subunits; heart tissue
- LDH2 - 3 heart & 1 muscle subunits; reticuloendothelial system
- LDH3 - 2 heart and 2 muscle subunits; lungs
- LDH4 - 1 heart and 3 muscle subunits; kidneys
- LDH5 - 4 muscle subunits; liver & skeletal muscle
Physiological functions:
- involved in the anaerobic metabolism of glucose when there is an absence or limited supply of oxygen in tissues
- catalyses the reversible conversion of pyruvate to lactate, with the use of NADH
- lactate is transported to the liver
- reversibly converting lactate to pyruvate
Clinical relevance:
- slight to moderate elevation: acute myocardial infarction, pulmonary embolism, leukaemia, haemolytic anaemia, liver & renal disease
- significantly elevated: pernicious anaemia, megaloblastic anaemia, certain cancers