Question

Why does LDH exist as a tetramer? What differences exist between LDHA and LDHB at the dimer interface? What would happen if LDH were monomeric?

Answer 1

Lactate dehydrogenase(LDH or LD) is an enzyme found in nearly all living cells i.e animals plants and all prokaryotes.LDH catalyzes the conversion of lactate to pyruvic acid and back as it converts NAD+ to NADH and back.

The two different subunits of LDH LDHA also known as the M subunit of LDH and LDHB also known as the H subunit of LDH both retain the same active site i.e the Histidine 193 active site and the same amino acids participating in the reaction .The noticeable difference between the two subunits is the replacement of alanine in the M chain with a glutamine in the H chain.This is the reason why H subunit can bind faster and the M subunit's catalytic activity is not reduced when subjected to the same conditions as the H subunit while H subunit's activity is reduced to a considerable degree.

There are two more subunits LDHC and LDHBx.LDHC is testes specific and LDHBx is a peroxisome specific LDH protein.Therefore due to these four subunits LDH exists as a tetramer.

Nature has decided a particular structure of an enzyme to work on a specified substrate.If LDH were monomeric the conversion from lactate to pyruvic acid in our body would not have been possible.