Charge of Tyrosine in a Protein (bounced)

TAO BRIAN T 90taobri at
Thu Jul 15 01:16:12 EST 1993

    My e-mail reply bounced, so I'm reposting it here...

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From: 90taobri at (TAO  BRIAN T)
Message-Id: <9307132126.AA27139 at>
To: TAFFETS at SNYSYRV1.bitnet
Subject: Re: Charge of Tyrosine in a Protein
Date: Tue Jul 13 17:08:19 1993 EST
Organization: MuGS Research and Development Facility
X-Mailer: MuGS 3.0d28 [Jul  8 93]

You previously wrote...
> We have recently been analyzing some results with site-directed
> mutations which can be most easily explained if a tyrosine in a protein
> we have been analyzing was negatively charged while the cells were at pH
> 7.0.  Is this possible (other than if the tyrosine was phosphorylated)
> and if it is possible can anyone suggest a reference on this?

    I'm at home right now, so I don't have any references handy, but I
don't see why tyrosine can't be deprotonated at pH 7.  Its standard pKa
is around 10, but check for a strong acceptor in the region of the OH
group which could pull off a proton.  For example, the chymotrypsin
protease mechanism involves a serine protonating an adjacent histidine
at physiological pH.  The negative serine is then stabilized by the
terminal carboxyl group of the substrate. If it can happen to serine
(normal pKa of around 14), it's conceivable that the pKa of tyrosine can
be lowered below 7.
Brian Tao:: taob at (Internex Online, 416-363-3783, 10 lines, v.32bis)
::::::::::: *** Note that my <taob at> address has changed ***

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