Protein Structure and Function Lecture 7 Notes: Glycosylation and Therapeutic Antibodies Glycation - Glycation is a purely chemical post translational modification that is not mediated by enzymes - N-glycosylation in the leg region of the SARS-COV-2 Spike protein might protect the leg region hinges from antibody binding and generating an inflammatory response, as well as keeping them flexible. - Glycation occurs when free sugars (e.g. glucose) interact with a protein's primary amino group (from the N-terminus or lysine residues). This process occurs via the open chain form of a free sugar. For example, the regular form of glucose in HO NH NH? solution is a cyclic form, however, in OH D-Glucose Primary Amine Schiff Base Amadori Product (i.e., Fructosyl-lysine) equilibrium less than 1% is present in the open chain form which is chemically an aldehyde. Aldehydes are reactive species which can react with an amino group and form a linkage between the glucose and protein. This linkage is not stable but can undergo a chemical rearrangement reaction known as the Amadori arrangement to become a more stable form (e.g. Fructosyl-lysine). OH CHO Protcin Protein CH OH + HC H `OH H Protein OH 0 - This reaction will occur where there are concentrations of free proteins and sugars, such as in plasma. The reaction is thought to be important in skin aging, diabetes and cancers (methylglyoxal). In this case, the reactive agent does not have to be glucose, it can be a smaller aldehyde such as methylglyoxal. - Although the process is purely chemical, the presence of these products has biological consequences. For example, fructoselysine is capable of further reaction through (mainly) oxidative processes to yield more products known as advanced glycation end products (AGEs). These can also have biological consequences, including: H N.H H-C-OH CH H.COH R Glucose Schiff Rase i???????? Amadori products Z NH NH- arg His Pontosidine HbA1c Glyoxal COOH CH2 -NH-R' CH2 NH Lipids IVs (a) Ageing (b) Diabetes (including vascular complications) (c) Atherosclerosis (d) Chronic kidney disease (e) Stroke (f) Cataract development (g) Asthma (h) Arthritis Carboxymethyl-lysine (CML) Furosine - An adduct (addition product) is a product of direct addition of two or more distinct molecules, resulting in a single reaction product (distinct molecular species) which contains all atoms of all components of the reaction.
- For example, Amadori adducts are involved in diabetes mellitus. The target (early glycated) proteins and their actions as causative agents in diabetes include: (a) Human serum albumin (causative agent in type 1 and type 2 diabetes with nephropathy and retinopathy) (b) Collagen (diabetic retinopathy) (c) Immunoglobulins (IgG/IgA/IgM, type 1 and 2 with nephropathy) (d) Plasma proteins (type 2) (e) Lipoproteins (low density lipoprotein/LDL, diabetic atherosclerosis) (f) Histone (diabetes) - Another protein that can be glycated is haemoglobin. This is a consequence of glucose content and haemoglobin in blood. Glucose binds to the N-terminal valine in the ß-chain of haemoglobin. The level of glycated Hb (HbA1c) gives an indication of the average blood glucose/diabetic status of an individual over the