asure the rate in the first few seconds of a reaction (V0) you are actually measuring the steady state. This is the rate used in Michaelis-Menten Kinetics.
Two terms that are important within Michaelis-Menten Kinetics are:
- Vmax – the maximum rate of reaction when all enzyme active sites are saturated with substrate
- Km – the substrate concentration that gives half maximal velocity. Km is a measure of the affinity an enzyme has for its substrate, as a lower Km means that less of the substrate is required to reach half of Vmax.
This equation concerns the steady state of an enzymatic reaction with one substrate, and is given by:
It describes how the initial rate of reaction, V0, is affected by the initial substrate concentration, [S]0. It only looks at the start of the reaction. This allows it to ignore the reverse reaction where substrate is formed from product. This is because at the start of the reaction there is no product present to become substrate.
The plot of rate against concentration has the shape of a rectangular hyperbola. However, a more useful representation of Michaelis–Menten kinetics is a graph called a Lineweaver–Burk plot. The equation used to generate this plot is given by:
This allows an easier interpretation of various quantities from the graph, such as the presence of an inhibitor. This is shown in figure 3, where an inhibitor decreases Km and therefore shifts the original line upwards.
Clinical Relevance – Plasma Enzyme Assays
Plasma enzyme assays can detect abnormal levels of enzymes in plasma. Levels may be abnormal due to leakage from damaged tissue or lack of functional enzyme production. The assay measures units of activity in a sample and so it will not measure non-functional enzyme.
If levels of an enzyme are raised it may indicate damage to the tissue that enzyme is normally found in.
If levels are low it may indicate that the enzyme is either non-functional, being produced slower or broken down quicker possibly due to a genetic abnormality.
|Enzyme||Location||Indication if raised||Indication if low|
M4 – liver, skeletal muscle, heart
MH3 – heart
H4 – kidney, erythrocytes
|Acute/chronic tissue damage
Degree of elevation can indicate extent of damage
Isoenzymes may help to localize this
|Aspartate transaminase||Widely distributed
Liver, heart, skeletal muscle, kidneys, brain, erythrocytes
|Degree of elevation can indicate extent of damage
Liver disease, kidney failure, gall bladder disease
|Pregnancy, diabetes, beriberi (vitamin B1 deficiency)|
|Alanine transaminase||Everywhere but predominantly in liver||Liver disease, viral hepatitis, bile duct problems|
|Alkaline phosphatase||Greatest concentration in prostate
Specific isoenzymes in prostate, liver, bone, kidney, intestine and placenta
|Creatine kinase||Various tissues
MM – skeletal muscle, heart
MB – heart
BB – brain, neurons, thyroid, kidney, intestine
|MM – skeletal muscle dystrophy
MB – myocardial infarction in last 2-3 days
BB – brain tumour
|Amylase||Saliva, exocrine pancreas||Pancreatitis|
|Acid phosphatase||Specific isoenzymes in liver, erythrocytes, platelets and bone||Diagnosis and treatment of prostate cancer|
|Cholinesterase||Blood plasma||Susceptibility to succinylcholine induced respiratory paralysis|