A. Arya (Madurai Kamraj University)

I hope the teaching sessions are being enjoyed by the students as they progress  more towards the final CSIR-NET June exam, we are constantly working off –classroom for strenghthenining your concepts and  thereby making you more confident. Here are some Biochemistry Questions asked in December 2014 CSIR-NET exam, with their solutions.

2.    In proteins, hydrogen bonds from as follows; Donor (D)-H--- Acceptor (A). Hydrogen bond is more favourable if the angle between D-H and A is
a)     90°              
b)    180°                           
c)    >180° 
d)    120°   

Solution – The highest bonding energy for the hydrogen bond is obtained, when the two negative charges are approximately in line with the positive charge in-between (O...H-O in water, O...H-N in the oxygen amide hydrogen bond). Deviations from linearity will reduce the bonding energy rapidly. It follows that the "best" hydrogen bond is "close" to linear.

Here the angle between the D-H bond and the H...A hydrogen bond should be close to 180° for a strong hydrogen bond. Therefore answer is (b)           
(Ref. J. Overington, et al. Proc. Roy. Soc. Biol Sci. 1990)

3.    Reaction products inhibit catalysis in enzymes by
a)    Covalently binding to the enzyme.
b)    Altering the enzyme structure.
c)    Occupying the active site.     
d)    From a complex with the substrate.

Solution – This mode of enzyme regulation is also known as feedback inhibition where the product of the reaction catalyzed by the reaction inhibits the reaction. Most commonly observed feedback inhibitions involve the binding of product with the enzyme at the same site as that of the substrate and inhibiting it in competitive mode, therefore (c) is the correct answer here. The binding is usually non-covalent.

4.      Glycosaminoglycans are usually linked to proteins to from proteoglycans. Which of the following is NOT a proteoglycan?
a)      Hyaluronan     
b)      Aggrecan
c)      Betaglycan      
d)      Syndecan-1

Solution – This is a straightforward memory based question, one must know that  Hyaluronan is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglycans in that it is nonsulfated, forms in the plasma membrane instead of the Golgi, and can be very large, with its molecular weight often reaching the millions.  Aggrecan is a proteoglycan, or a protein modified with large carbohydrates; the human form of the protein is 2316 amino acids long and can be expressed in multiple isoforms due to alternative splicing.  Betaglycan is Membrane-anchored and Soluble Forms of Betaglycan, a Polymorphic. Proteoglycan that Binds Transforming Growth Factor,Syndecans are Single transmembrane domain proteins that are thought to act as coreceptors, especially for G protein-coupled receptors. These core proteins carry three to five heparan sulfate and chondroitin sulfate chains, which allow for interaction with a large variety of ligands including fibroblast growth factors, vascular endothelial growth factor, transforming growth factor-beta, fibronectin and antithrombin-1. Therefore (d) is the correct answer here, which is a membrane protein.

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